replay.c, replay.h

diff --git a/replay.c b/replay.c
index f96f1f6551..4ef8abb607 100644
--- a/replay.c
+++ b/replay.c
@@ -269,7 +269,8 @@ static struct commit *pick_regular_commit(struct repository *repo,
 					  struct commit *onto,
 					  struct merge_options *merge_opt,
 					  struct merge_result *result,
-					  enum replay_mode mode)
+					  enum replay_mode mode,
+					  enum replay_empty_commit_action empty)
 {
 	struct commit *base, *replayed_base;
 	struct tree *pickme_tree, *base_tree, *replayed_base_tree;
@@ -321,12 +322,25 @@ static struct commit *pick_regular_commit(struct repository *repo,
 	}
 	merge_opt->ancestor = NULL;
 	merge_opt->branch2 = NULL;
+
 	if (!result->clean)
 		return NULL;
-	/* Drop commits that become empty */
+
+	/* Handle commits that become empty */
 	if (oideq(&replayed_base_tree->object.oid, &result->tree->object.oid) &&
-	    !oideq(&pickme_tree->object.oid, &base_tree->object.oid))
-		return replayed_base;
+	    !oideq(&pickme_tree->object.oid, &base_tree->object.oid)) {
+		switch (empty) {
+		case REPLAY_EMPTY_COMMIT_DROP:
+			return replayed_base;
+		case REPLAY_EMPTY_COMMIT_KEEP:
+			break;
+		case REPLAY_EMPTY_COMMIT_ABORT:
+			result->clean = error(_("commit %s became empty after replay"),
+					      oid_to_hex(&pickme->object.oid));
+			return NULL;
+		}
+	}
+
 	return create_commit(repo, result->tree, pickme, replayed_base, mode);
 }
 
@@ -417,7 +431,7 @@ int replay_revisions(struct rev_info *revs,
 
 		last_commit = pick_regular_commit(revs->repo, commit, replayed_commits,
 						  mode == REPLAY_MODE_REVERT ? last_commit : onto,
-						  &merge_opt, &result, mode);
+						  &merge_opt, &result, mode, opts->empty);
 		if (!last_commit)
 			break;
 
@@ -458,6 +472,11 @@ int replay_revisions(struct rev_info *revs,
 		}
 	}
 
+	if (result.clean < 0) {
+		ret = -1;
+		goto out;
+	}
+
 	if (!result.clean) {
 		ret = 1;
 		goto out;
diff --git a/replay.h b/replay.h
index 0ab74b9805..1851a07705 100644
--- a/replay.h
+++ b/replay.h
@@ -6,6 +6,19 @@
 struct repository;
 struct rev_info;
 
+/*
+ * Controls what happens when a replayed commit becomes empty (i.e. its tree
+ * is identical to its parent's tree after the replay).
+ */
+enum replay_empty_commit_action {
+	/* Silently discard the empty commit. */
+	REPLAY_EMPTY_COMMIT_DROP,
+	/* Keep the empty commit as-is. */
+	REPLAY_EMPTY_COMMIT_KEEP,
+	/* Abort with an error. */
+	REPLAY_EMPTY_COMMIT_ABORT,
+};
+
 /*
  * A set of options that can be passed to `replay_revisions()`.
  */
@@ -43,6 +56,12 @@ struct replay_revisions_options {
 	 * Requires `onto` to be set.
 	 */
 	int contained;
+
+	/*
+	 * Controls what to do when a replayed commit becomes empty.
+	 * Defaults to REPLAY_EMPTY_COMMIT_DROP.
+	 */
+	enum replay_empty_commit_action empty;
 };
 
 /* This struct is used as an out-parameter by `replay_revisions()`. */

diff --git a/builtin/history.c b/builtin/history.c
index 9526938085..549e352c74 100644
--- a/builtin/history.c
+++ b/builtin/history.c
@@ -91,13 +91,18 @@ static int fill_commit_message(struct repository *repo,
 	return 0;
 }
 
-static int commit_tree_with_edited_message_ext(struct repository *repo,
-					       const char *action,
-					       struct commit *commit_with_message,
-					       const struct commit_list *parents,
-					       const struct object_id *old_tree,
-					       const struct object_id *new_tree,
-					       struct commit **out)
+enum commit_tree_flags {
+	COMMIT_TREE_EDIT_MESSAGE = (1 << 0),
+};
+
+static int commit_tree_ext(struct repository *repo,
+			   const char *action,
+			   struct commit *commit_with_message,
+			   const struct commit_list *parents,
+			   const struct object_id *old_tree,
+			   const struct object_id *new_tree,
+			   struct commit **out,
+			   enum commit_tree_flags flags)
 {
 	const char *exclude_gpgsig[] = {
 		/* We reencode the message, so the encoding needs to be stripped. */
@@ -122,10 +127,14 @@ static int commit_tree_with_edited_message_ext(struct repository *repo,
 		original_author = xmemdupz(ptr, len);
 	find_commit_subject(original_message, &original_body);
 
-	ret = fill_commit_message(repo, old_tree, new_tree,
-				  original_body, action, &commit_message);
-	if (ret < 0)
-		goto out;
+	if (flags & COMMIT_TREE_EDIT_MESSAGE) {
+		ret = fill_commit_message(repo, old_tree, new_tree,
+					  original_body, action, &commit_message);
+		if (ret < 0)
+			goto out;
+	} else {
+		strbuf_addstr(&commit_message, original_body);
+	}
 
 	original_extra_headers = read_commit_extra_headers(commit_with_message,
 							   exclude_gpgsig);
@@ -168,8 +177,8 @@ static int commit_tree_with_edited_message(struct repository *repo,
 		oidcpy(&parent_tree_oid, repo->hash_algo->empty_tree);
 	}
 
-	return commit_tree_with_edited_message_ext(repo, action, original, original->parents,
-						   &parent_tree_oid, tree_oid, out);
+	return commit_tree_ext(repo, action, original, original->parents,
+			       &parent_tree_oid, tree_oid, out, COMMIT_TREE_EDIT_MESSAGE);
 }
 
 enum ref_action {
@@ -616,9 +625,8 @@ static int split_commit(struct repository *repo,
 	 * The first commit is constructed from the split-out tree. The base
 	 * that shall be diffed against is the parent of the original commit.
 	 */
-	ret = commit_tree_with_edited_message_ext(repo, "split-out", original,
-						  original->parents, &parent_tree_oid,
-						  &split_tree->object.oid, &first_commit);
+	ret = commit_tree_ext(repo, "split-out", original, original->parents, &parent_tree_oid,
+			      &split_tree->object.oid, &first_commit, COMMIT_TREE_EDIT_MESSAGE);
 	if (ret < 0) {
 		ret = error(_("failed writing first commit"));
 		goto out;
@@ -634,9 +642,8 @@ static int split_commit(struct repository *repo,
 	old_tree_oid = &repo_get_commit_tree(repo, first_commit)->object.oid;
 	new_tree_oid = &repo_get_commit_tree(repo, original)->object.oid;
 
-	ret = commit_tree_with_edited_message_ext(repo, "split-out", original,
-						  parents, old_tree_oid,
-						  new_tree_oid, &second_commit);
+	ret = commit_tree_ext(repo, "split-out", original, parents, old_tree_oid,
+			      new_tree_oid, &second_commit, COMMIT_TREE_EDIT_MESSAGE);
 	if (ret < 0) {
 		ret = error(_("failed writing second commit"));
 		goto out;

Documentation/git-history.adoc, builtin/history.c, t/meson.build, t/t3453-history-fixup.sh

diff --git a/Documentation/git-history.adoc b/Documentation/git-history.adoc
index 24dc907033..6576379f77 100644
--- a/Documentation/git-history.adoc
+++ b/Documentation/git-history.adoc
@@ -8,6 +8,7 @@ git-history - EXPERIMENTAL: Rewrite history
 SYNOPSIS
 --------
 [synopsis]
+git history fixup <commit> [--dry-run] [--update-refs=(branches|head)] [--reedit-message] [--empty=(drop|keep|abort)]
 git history reword <commit> [--dry-run] [--update-refs=(branches|head)]
 git history split <commit> [--dry-run] [--update-refs=(branches|head)] [--] [<pathspec>...]
 
@@ -22,8 +23,9 @@ THIS COMMAND IS EXPERIMENTAL. THE BEHAVIOR MAY CHANGE.
 This command is related to linkgit:git-rebase[1] in that both commands can be
 used to rewrite history. There are a couple of major differences though:
 
-* linkgit:git-history[1] can work in a bare repository as it does not need to
-  touch either the index or the worktree.
+* Most subcommands of linkgit:git-history[1] can work in a bare repository as
+  they do not need to touch either the index or the worktree. The `fixup`
+  subcommand is an exception to this, as it reads staged changes from the index.
 * linkgit:git-history[1] does not execute any linkgit:githooks[5] at the
   current point in time. This may change in the future.
 * linkgit:git-history[1] by default updates all branches that are descendants
@@ -48,11 +50,27 @@ conflicts. This limitation is by design as history rewrites are not intended to
 be stateful operations. The limitation can be lifted once (if) Git learns about
 first-class conflicts.
 
+When using `fixup` with `--empty=drop`, dropping the root commit is not yet
+supported.
+
 COMMANDS
 --------
 
 The following commands are available to rewrite history in different ways:
 
+`fixup <commit>`::
+	Apply the currently staged changes to the specified commit. This
+	is done by performing a three-way merge between the HEAD commit,
+	the target commit and the tree generated from staged changes.
+	This is using the same logic as linkgit:git-cherry-pick[1].
++
+The commit message and authorship of the target commit are preserved by
+default, unless you specify `--reedit-message`.
++
+If applying the staged changes would result in a conflict, the command
+aborts with an error. All branches that are descendants of the original
+commit are updated to point to the rewritten history.
+
 `reword <commit>`::
 	Rewrite the commit message of the specified commit. All the other
 	details of this commit remain unchanged. This command will spawn an
@@ -87,6 +105,31 @@ OPTIONS
 	objects will be written into the repository, so applying these printed
 	ref updates is generally safe.
 
+`--reedit-message`::
+	Open an editor to modify the target commit's message.
+
+`--empty=(drop|keep|abort)`::
+	Control what happens when a commit becomes empty as a result of the
+	fixup. This can happen in two situations:
++
+--
+* The fixup target itself becomes empty because the staged changes exactly
+  cancel out all changes introduced by that commit.
+
+* A descendant commit becomes empty during replay because it introduced the
+  same change that was just fixed up into an ancestor.
+--
++
+With `drop` (the default), empty commits are removed from the rewritten
+history. Descendants of a dropped target commit are replayed directly onto
+the target's parent. Note that dropping the root commit is not supported;
+see LIMITATIONS.
++
+With `keep`, empty commits are retained in the rewritten history as-is.
++
+With `abort`, the command stops with an error if any commit would become
+empty.
+
 `--update-refs=(branches|head)`::
 	Control which references will be updated by the command, if any. With
 	`branches`, all local branches that point to commits which are
@@ -96,6 +139,36 @@ OPTIONS
 EXAMPLES
 --------
 
+Fixup a commit
+~~~~~~~~~~~~~~
+
+----------
+$ git log --oneline --stat
+abc1234 (HEAD -> main) third
+ third.txt | 1 +
+def5678 second
+ second.txt | 1 +
+ghi9012 first
+ first.txt | 1 +
+
+$ echo "change" >>unrelated.txt
+$ git add unrelated.txt
+$ git history fixup ghi9012
+
+$ git log --oneline --stat
+jkl3456 (HEAD -> main) third
+ third.txt | 1 +
+mno7890 second
+ second.txt | 1 +
+pqr1234 first
+ first.txt     | 1 +
+ unrelated.txt | 1 +
+----------
+
+The staged addition of `unrelated.txt` has been incorporated into the `first`
+commit. All descendant commits have been replayed on top of the rewritten
+history.
+
 Split a commit
 ~~~~~~~~~~~~~~
 
diff --git a/builtin/history.c b/builtin/history.c
index 549e352c74..0fc06fb204 100644
--- a/builtin/history.c
+++ b/builtin/history.c
@@ -10,6 +10,7 @@
 #include "gettext.h"
 #include "hex.h"
 #include "lockfile.h"
+#include "merge-ort.h"
 #include "oidmap.h"
 #include "parse-options.h"
 #include "path.h"
@@ -23,6 +24,8 @@
 #include "unpack-trees.h"
 #include "wt-status.h"
 
+#define GIT_HISTORY_FIXUP_USAGE \
+	N_("git history fixup <commit> [--dry-run] [--update-refs=(branches|head)] [--reedit-message] [--empty=(drop|keep|abort)]")
 #define GIT_HISTORY_REWORD_USAGE \
 	N_("git history reword <commit> [--dry-run] [--update-refs=(branches|head)]")
 #define GIT_HISTORY_SPLIT_USAGE \
@@ -335,10 +338,13 @@ static int handle_reference_updates(struct rev_info *revs,
 				    struct commit *original,
 				    struct commit *rewritten,
 				    const char *reflog_msg,
-				    int dry_run)
+				    int dry_run,
+				    enum replay_empty_commit_action empty)
 {
 	const struct name_decoration *decoration;
-	struct replay_revisions_options opts = { 0 };
+	struct replay_revisions_options opts = {
+		.empty = empty,
+	};
 	struct replay_result result = { 0 };
 	struct ref_transaction *transaction = NULL;
 	struct strbuf err = STRBUF_INIT;
@@ -434,6 +440,236 @@ static int handle_reference_updates(struct rev_info *revs,
 	return ret;
 }
 
+static int commit_became_empty(struct repository *repo,
+			       struct commit *original,
+			       struct tree *result)
+{
+	struct commit *parent = original->parents ? original->parents->item : NULL;
+	struct object_id parent_tree_oid;
+
+	if (parent) {
+		if (repo_parse_commit(repo, parent))
+			return error(_("unable to parse parent of %s"),
+				     oid_to_hex(&original->object.oid));
+
+		parent_tree_oid = repo_get_commit_tree(repo, parent)->object.oid;
+	} else {
+		oidcpy(&parent_tree_oid, repo->hash_algo->empty_tree);
+	}
+
+	return oideq(&result->object.oid, &parent_tree_oid);
+}
+
+static int parse_opt_empty(const struct option *opt, const char *arg, int unset)
+{
+	enum replay_empty_commit_action *value = opt->value;
+
+	BUG_ON_OPT_NEG(unset);
+
+	if (!strcmp(arg, "drop"))
+		*value = REPLAY_EMPTY_COMMIT_DROP;
+	else if (!strcmp(arg, "keep"))
+		*value = REPLAY_EMPTY_COMMIT_KEEP;
+	else if (!strcmp(arg, "abort"))
+		*value = REPLAY_EMPTY_COMMIT_ABORT;
+	else
+		die(_("unrecognized '--empty=' action '%s'; "
+		      "valid values are \"drop\", \"keep\", and \"abort\"."), arg);
+
+	return 0;
+}
+
+static int cmd_history_fixup(int argc,
+			     const char **argv,
+			     const char *prefix,
+			     struct repository *repo)
+{
+	const char * const usage[] = {
+		GIT_HISTORY_FIXUP_USAGE,
+		NULL,
+	};
+	enum replay_empty_commit_action empty = REPLAY_EMPTY_COMMIT_DROP;
+	enum ref_action action = REF_ACTION_DEFAULT;
+	enum commit_tree_flags flags = 0;
+	int dry_run = 0;
+	struct option options[] = {
+		OPT_CALLBACK_F(0, "update-refs", &action, "(branches|head)",
+			       N_("control which refs should be updated"),
+			       PARSE_OPT_NONEG, parse_ref_action),
+		OPT_BOOL('n', "dry-run", &dry_run,
+			 N_("perform a dry-run without updating any refs")),
+		OPT_BIT(0, "reedit-message", &flags,
+			N_("open an editor to modify the commit message"),
+			COMMIT_TREE_EDIT_MESSAGE),
+		OPT_CALLBACK_F(0, "empty", &empty, "(drop|keep|abort)",
+			       N_("how to handle commits that become empty"),
+			       PARSE_OPT_NONEG, parse_opt_empty),
+		OPT_END(),
+	};
+	struct merge_result merge_result = { 0 };
+	struct merge_options merge_opts = { 0 };
+	struct strbuf reflog_msg = STRBUF_INIT;
+	struct commit *head_commit, *original, *rewritten;
+	struct tree *head_tree, *original_tree, *index_tree;
+	struct rev_info revs = { 0 };
+	bool skip_commit = false;
+	int ret;
+
+	argc = parse_options(argc, argv, prefix, options, usage, 0);
+	if (argc != 1) {
+		ret = error(_("command expects a single revision"));
+		goto out;
+	}
+	repo_config(repo, git_default_config, NULL);
+
+	if (action == REF_ACTION_DEFAULT)
+		action = REF_ACTION_BRANCHES;
+
+	if (is_bare_repository()) {
+		ret = error(_("cannot run fixup in a bare repository"));
+		goto out;
+	}
+
+	/* Resolve the original commit, which is the one we want to fix up. */
+	original = lookup_commit_reference_by_name(argv[0]);
+	if (!original) {
+		ret = error(_("commit cannot be found: %s"), argv[0]);
+		goto out;
+	}
+
+	/*
+	 * Resolve HEAD so we can use its tree as the merge base: the staged
+	 * changes are expressed as a diff from HEAD's tree to the index tree.
+	 */
+	head_commit = lookup_commit_reference_by_name("HEAD");
+	if (!head_commit) {
+		ret = error(_("cannot look up HEAD"));
+		goto out;
+	}
+
+	head_tree = repo_get_commit_tree(repo, head_commit);
+	if (!head_tree) {
+		ret = error(_("cannot get tree for HEAD"));
+		goto out;
+	}
+
+	if (repo_read_index(repo) < 0) {
+		ret = error(_("unable to read index"));
+		goto out;
+	}
+
+	if (!repo_index_has_changes(repo, head_tree, NULL)) {
+		ret = error(_("nothing to fixup: no staged changes"));
+		goto out;
+	}
+
+	/*
+	 * Write the index as a tree object. This is the "theirs" side of the
+	 * three-way merge: it is HEAD's tree with the staged changes applied.
+	 */
+	index_tree = write_in_core_index_as_tree(repo, repo->index);
+	if (!index_tree) {
+		ret = error(_("unable to write index as a tree"));
+		goto out;
+	}
+
+	original_tree = repo_get_commit_tree(repo, original);
+	if (!original_tree) {
+		ret = error(_("cannot get tree for commit %s"), argv[0]);
+		goto out;
+	}
+
+	/*
+	 * Perform the three-way merge to reapply changes in the index onto the
+	 * target commit. This is using basically the same logic as a
+	 * cherry-pick, where the base commit is our HEAD, ours is the original
+	 * tree and theirs is the index tree.
+	 */
+	init_basic_merge_options(&merge_opts, repo);
+	merge_opts.ancestor = "HEAD";
+	merge_opts.branch1 = argv[0];
+	merge_opts.branch2 = "staged";
+	merge_incore_nonrecursive(&merge_opts, head_tree,
+				  original_tree, index_tree, &merge_result);
+
+	if (merge_result.clean < 0) {
+		ret = error(_("merge failed while applying fixup"));
+		goto out;
+	}
+
+	if (!merge_result.clean) {
+		ret = error(_("fixup would produce conflicts; aborting"));
+		goto out;
+	}
+
+	ret = commit_became_empty(repo, original, merge_result.tree);
+	if (ret < 0)
+		goto out;
+	if (ret > 0) {
+		switch (empty) {
+		case REPLAY_EMPTY_COMMIT_DROP:
+			/*
+			 * Drop the target commit by replaying its descendants
+			 * directly onto its parent.
+			 */
+			rewritten = original->parents ? original->parents->item : NULL;
+
+			/*
+			 * TODO: we don't yet have the ability to drop root
+			 * commits, but there's ultimately no good reason for
+			 * this restriction to exist other than a technical
+			 * limitation.
+			 */
+			if (!rewritten) {
+				ret = error(_("cannot drop root commit %s: "
+					      "it has no parent to replay onto"),
+					    argv[0]);
+				goto out;
+			}
+
+			skip_commit = true;
+			break;
+		case REPLAY_EMPTY_COMMIT_KEEP:
+			/* Proceed and record the empty commit. */
+			break;
+		case REPLAY_EMPTY_COMMIT_ABORT:
+			ret = error(_("fixup makes commit %s empty"), argv[0]);
+			goto out;
+		}
+	}
+
+	ret = setup_revwalk(repo, action, original, &revs);
+	if (ret)
+		goto out;
+
+	if (!skip_commit) {
+		ret = commit_tree_ext(repo, "fixup", original, original->parents,
+				      &original_tree->object.oid, &merge_result.tree->object.oid,
+				      &rewritten, flags);
+		if (ret < 0) {
+			ret = error(_("failed writing fixed-up commit"));
+			goto out;
+		}
+	}
+
+	strbuf_addf(&reflog_msg, "fixup: updating %s", argv[0]);
+
+	ret = handle_reference_updates(&revs, action, original, rewritten,
+				       reflog_msg.buf, dry_run, empty);
+	if (ret < 0) {
+		ret = error(_("failed replaying descendants"));
+		goto out;
+	}
+
+	ret = 0;
+
+out:
+	merge_finalize(&merge_opts, &merge_result);
+	strbuf_release(&reflog_msg);
+	release_revisions(&revs);
+	return ret;
+}
+
 static int cmd_history_reword(int argc,
 			      const char **argv,
 			      const char *prefix,
@@ -487,7 +723,7 @@ static int cmd_history_reword(int argc,
 	strbuf_addf(&reflog_msg, "reword: updating %s", argv[0]);
 
 	ret = handle_reference_updates(&revs, action, original, rewritten,
-				       reflog_msg.buf, dry_run);
+				       reflog_msg.buf, dry_run, REPLAY_EMPTY_COMMIT_ABORT);
 	if (ret < 0) {
 		ret = error(_("failed replaying descendants"));
 		goto out;
@@ -724,7 +960,7 @@ static int cmd_history_split(int argc,
 	strbuf_addf(&reflog_msg, "split: updating %s", argv[0]);
 
 	ret = handle_reference_updates(&revs, action, original, rewritten,
-				       reflog_msg.buf, dry_run);
+				       reflog_msg.buf, dry_run, REPLAY_EMPTY_COMMIT_ABORT);
 	if (ret < 0) {
 		ret = error(_("failed replaying descendants"));
 		goto out;
@@ -745,12 +981,14 @@ int cmd_history(int argc,
 		struct repository *repo)
 {
 	const char * const usage[] = {
+		GIT_HISTORY_FIXUP_USAGE,
 		GIT_HISTORY_REWORD_USAGE,
 		GIT_HISTORY_SPLIT_USAGE,
 		NULL,
 	};
 	parse_opt_subcommand_fn *fn = NULL;
 	struct option options[] = {
+		OPT_SUBCOMMAND("fixup", &fn, cmd_history_fixup),
 		OPT_SUBCOMMAND("reword", &fn, cmd_history_reword),
 		OPT_SUBCOMMAND("split", &fn, cmd_history_split),
 		OPT_END(),
diff --git a/t/meson.build b/t/meson.build
index 7528e5cda5..f502ad8ec9 100644
--- a/t/meson.build
+++ b/t/meson.build
@@ -397,6 +397,7 @@ integration_tests = [
   't3450-history.sh',
   't3451-history-reword.sh',
   't3452-history-split.sh',
+  't3453-history-fixup.sh',
   't3500-cherry.sh',
   't3501-revert-cherry-pick.sh',
   't3502-cherry-pick-merge.sh',
diff --git a/t/t3453-history-fixup.sh b/t/t3453-history-fixup.sh
new file mode 100755
index 0000000000..868298e248
--- /dev/null
+++ b/t/t3453-history-fixup.sh
@@ -0,0 +1,680 @@
+#!/bin/sh
+
+test_description='tests for git-history fixup subcommand'
+
+. ./test-lib.sh
+
+fixup_with_message () {
+	cat >message &&
+	write_script fake-editor.sh <<-\EOF &&
+	cp message "$1"
+	EOF
+	test_set_editor "$(pwd)"/fake-editor.sh &&
+	git history fixup --reedit-message "$@" &&
+	rm fake-editor.sh message
+}
+
+expect_changes () {
+	git log --format="%s" --numstat "$@" >actual.raw &&
+	sed '/^$/d' <actual.raw >actual &&
+	cat >expect &&
+	test_cmp expect actual
+}
+
+test_expect_success 'errors on missing commit argument' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		test_must_fail git history fixup 2>err &&
+		test_grep "command expects a single revision" err
+	)
+'
+
+test_expect_success 'errors on too many arguments' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		test_must_fail git history fixup HEAD HEAD 2>err &&
+		test_grep "command expects a single revision" err
+	)
+'
+
+test_expect_success 'errors on unknown revision' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		test_must_fail git history fixup does-not-exist 2>err &&
+		test_grep "commit cannot be found: does-not-exist" err
+	)
+'
+
+test_expect_success 'errors when nothing is staged' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		test_must_fail git history fixup HEAD 2>err &&
+		test_grep "nothing to fixup: no staged changes" err
+	)
+'
+
+test_expect_success 'errors in a bare repository' '
+	test_when_finished "rm -rf repo repo.git" &&
+	git init repo &&
+	test_commit -C repo initial &&
+	git clone --bare repo repo.git &&
+	test_must_fail git -C repo.git history fixup HEAD 2>err &&
+	test_grep "cannot run fixup in a bare repository" err
+'
+
+test_expect_success 'errors with invalid --empty= value' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	test_must_fail git -C repo history fixup --empty=bogus HEAD 2>err &&
+	test_grep "unrecognized.*--empty.*bogus" err
+'
+
+test_expect_success 'can fixup the tip commit' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		echo content >file.txt &&
+		git add file.txt &&
+		git commit -m "add file" &&
+
+		echo fix >>file.txt &&
+		git add file.txt &&
+
+		expect_changes <<-\EOF &&
+		add file
+		1	0	file.txt
+		initial
+		1	0	initial.t
+		EOF
+
+		git symbolic-ref HEAD >branch-expect &&
+		git history fixup HEAD &&
+		git symbolic-ref HEAD >branch-actual &&
+		test_cmp branch-expect branch-actual &&
+
+		expect_changes <<-\EOF &&
+		add file
+		2	0	file.txt
+		initial
+		1	0	initial.t
+		EOF
+
+		# Verify the fix is in the tip commit tree
+		git show HEAD:file.txt >actual &&
+		printf "content\nfix\n" >expect &&
+		test_cmp expect actual &&
+
+		git reflog >reflog &&
+		test_grep "fixup: updating HEAD" reflog
+	)
+'
+
+test_expect_success 'can fixup a commit in the middle of history' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit first &&
+		echo content >file.txt &&
+		git add file.txt &&
+		git commit -m "add file" &&
+		test_commit third &&
+
+		echo fix >>file.txt &&
+		git add file.txt &&
+
+		expect_changes <<-\EOF &&
+		third
+		1	0	third.t
+		add file
+		1	0	file.txt
+		first
+		1	0	first.t
+		EOF
+
+		git history fixup HEAD~ &&
+
+		expect_changes <<-\EOF &&
+		third
+		1	0	third.t
+		add file
+		2	0	file.txt
+		first
+		1	0	first.t
+		EOF
+
+		# Verify the fix landed in the "add file" commit.
+		git show HEAD~:file.txt >actual &&
+		printf "content\nfix\n" >expect &&
+		test_cmp expect actual &&
+
+		# And verify that the replayed commit also has the change.
+		git show HEAD:file.txt >actual &&
+		printf "content\nfix\n" >expect &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'can fixup root commit' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		echo initial >root.txt &&
+		git add root.txt &&
+		git commit -m "root" &&
+		test_commit second &&
+
+		expect_changes <<-\EOF &&
+		second
+		1	0	second.t
+		root
+		1	0	root.txt
+		EOF
+
+		echo fix >>root.txt &&
+		git add root.txt &&
+		git history fixup HEAD~ &&
+
+		expect_changes <<-\EOF &&
+		second
+		1	0	second.t
+		root
+		2	0	root.txt
+		EOF
+
+		git show HEAD~:root.txt >actual &&
+		printf "initial\nfix\n" >expect &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'preserves commit message and authorship' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		echo content >file.txt &&
+		git add file.txt &&
+		git commit --author="Original <original@example.com>" -m "original message" &&
+
+		echo fix >>file.txt &&
+		git add file.txt &&
+		git history fixup HEAD &&
+
+		# Message preserved
+		git log -1 --format="%s" >actual &&
+		echo "original message" >expect &&
+		test_cmp expect actual &&
+
+		# Authorship preserved
+		git log -1 --format="%an <%ae>" >actual &&
+		echo "Original <original@example.com>" >expect &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'updates all descendant branches by default' '
+	test_when_finished "rm -rf repo" &&
+	git init repo --initial-branch=main &&
+	(
+		cd repo &&
+		test_commit base &&
+		git branch branch &&
+		test_commit ours &&
+		git switch branch &&
+		test_commit theirs &&
+		git switch main &&
+
+		echo fix >fix.txt &&
+		git add fix.txt &&
+		git history fixup base &&
+
+		expect_changes --branches <<-\EOF &&
+		theirs
+		1	0	theirs.t
+		ours
+		1	0	ours.t
+		base
+		1	0	base.t
+		1	0	fix.txt
+		EOF
+
+		# Both branches should have the fix in the base
+		git show main~:fix.txt >actual &&
+		echo fix >expect &&
+		test_cmp expect actual &&
+		git show branch~:fix.txt >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'can fixup commit on a different branch' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit base &&
+		git branch theirs &&
+		test_commit ours &&
+		git switch theirs &&
+		test_commit theirs &&
+
+		# Stage a change while on "theirs"
+		echo fix >fix.txt &&
+		git add fix.txt &&
+
+		# Ensure that "ours" does not change, as it does not contain
+		# the commit in question.
+		git rev-parse ours >ours-before &&
+		git history fixup theirs &&
+		git rev-parse ours >ours-after &&
+		test_cmp ours-before ours-after &&
+
+		git show HEAD:fix.txt >actual &&
+		echo fix >expect &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success '--dry-run prints ref updates without modifying repo' '
+	test_when_finished "rm -rf repo" &&
+	git init repo --initial-branch=main &&
+	(
+		cd repo &&
+		test_commit base &&
+		git branch branch &&
+		test_commit main-tip &&
+		git switch branch &&
+		test_commit branch-tip &&
+		git switch main &&
+
+		echo fix >fix.txt &&
+		git add fix.txt &&
+
+		git refs list >refs-before &&
+		git history fixup --dry-run base >updates &&
+		git refs list >refs-after &&
+		test_cmp refs-before refs-after &&
+
+		test_grep "update refs/heads/main" updates &&
+		test_grep "update refs/heads/branch" updates &&
+
+		expect_changes --branches <<-\EOF &&
+		branch-tip
+		1	0	branch-tip.t
+		main-tip
+		1	0	main-tip.t
+		base
+		1	0	base.t
+		EOF
+
+		git update-ref --stdin <updates &&
+		expect_changes --branches <<-\EOF
+		branch-tip
+		1	0	branch-tip.t
+		main-tip
+		1	0	main-tip.t
+		base
+		1	0	base.t
+		1	0	fix.txt
+		EOF
+	)
+'
+
+test_expect_success '--update-refs=head updates only HEAD' '
+	test_when_finished "rm -rf repo" &&
+	git init repo --initial-branch=main &&
+	(
+		cd repo &&
+		test_commit base &&
+		git branch branch &&
+		test_commit main-tip &&
+		git switch branch &&
+		test_commit branch-tip &&
+
+		echo fix >fix.txt &&
+		git add fix.txt &&
+
+		# Only HEAD (branch) should be updated
+		git history fixup --update-refs=head base &&
+
+		# The main branch should be unaffected.
+		expect_changes main <<-\EOF &&
+		main-tip
+		1	0	main-tip.t
+		base
+		1	0	base.t
+		EOF
+
+		# But the currently checked out branch should be modified.
+		expect_changes branch <<-\EOF
+		branch-tip
+		1	0	branch-tip.t
+		base
+		1	0	base.t
+		1	0	fix.txt
+		EOF
+	)
+'
+
+test_expect_success '--update-refs=head refuses to rewrite commits not in HEAD ancestry' '
+	test_when_finished "rm -rf repo" &&
+	git init repo --initial-branch=main &&
+	(
+		cd repo &&
+		test_commit base &&
+		git branch other &&
+		test_commit main-tip &&
+		git switch other &&
+		test_commit other-tip &&
+
+		echo fix >fix.txt &&
+		git add fix.txt &&
+
+		test_must_fail git history fixup --update-refs=head main-tip 2>err &&
+		test_grep "rewritten commit must be an ancestor of HEAD" err
+	)
+'
+
+test_expect_success 'aborts when fixup would produce conflicts' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		echo "line one" >file.txt &&
+		git add file.txt &&
+		git commit -m "first" &&
+
+		echo "line two" >file.txt &&
+		git add file.txt &&
+		git commit -m "second" &&
+
+		echo "conflicting change" >file.txt &&
+		git add file.txt &&
+
+		git refs list >refs-before &&
+		test_must_fail git history fixup HEAD~ 2>err &&
+		test_grep "fixup would produce conflicts" err &&
+		git refs list >refs-after &&
+		test_cmp refs-before refs-after
+	)
+'
+
+test_expect_success '--reedit-message opens editor for the commit message' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		test_commit initial &&
+		echo content >file.txt &&
+		git add file.txt &&
+		git commit -m "add file" &&
+
+		echo fix >>file.txt &&
+		git add file.txt &&
+
+		fixup_with_message HEAD <<-\EOF &&
+		add file with fix
+		EOF
+
+		expect_changes --branches <<-\EOF
+		add file with fix
+		2	0	file.txt
+		initial
+		1	0	initial.t
+		EOF
+	)
+'
+
+test_expect_success 'retains unstaged working tree changes after fixup' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+		touch a b &&
+		git add . &&
+		git commit -m "initial commit" &&
+		echo staged >a &&
+		echo unstaged >b &&
+		git add a &&
+		git history fixup HEAD &&
+
+		# b is still modified in the worktree but not staged
+		cat >expect <<-\EOF &&
+		 M b
+		EOF
+		git status --porcelain --untracked-files=no >actual &&
+		test_cmp expect actual
+	)
+'
+
+test_expect_success 'index is clean after fixup when target is HEAD' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		test_commit initial &&
+		echo fix >fix.txt &&
+		git add fix.txt &&
+		git history fixup HEAD &&
+
+		git status --porcelain --untracked-files=no >actual &&
+		test_must_be_empty actual
+	)
+'
+
+test_expect_success 'index is unchanged on conflict' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		echo base >file.txt &&
+		git add file.txt &&
+		git commit -m base &&
+		echo change >file.txt &&
+		git add file.txt &&
+		git commit -m change &&
+
+		echo conflict >file.txt &&
+		git add file.txt &&
+
+		git diff --cached >index-before &&
+		test_must_fail git history fixup HEAD~ &&
+		git diff --cached >index-after &&
+		test_cmp index-before index-after
+	)
+'
+
+test_expect_success '--empty=drop removes target commit and replays descendants onto its parent' '
+	test_when_finished "rm -rf repo" &&
+	git init repo --initial-branch=main &&
+	(
+		cd repo &&
+
+		test_commit first &&
+		test_commit second &&
+		test_commit third &&
+
+		git rm second.t &&
+		git history fixup --empty=drop HEAD~ &&
+
+		expect_changes <<-\EOF &&
+		third
+		1	0	third.t
+		first
+		1	0	first.t
+		EOF
+		test_must_fail git show HEAD:second.t
+	)
+'
+
+test_expect_success '--empty=drop errors out when dropping the root commit' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		test_commit first &&
+		test_commit second &&
+
+		git rm first.t &&
+		test_must_fail git history fixup --empty=drop HEAD~ 2>err &&
+		test_grep "cannot drop root commit" err
+	)
+'
+
+test_expect_success '--empty=drop can drop the HEAD commit' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		test_commit first &&
+		test_commit second &&
+
+		git rm second.t &&
+		git history fixup --empty=drop HEAD &&
+
+		expect_changes <<-\EOF
+		first
+		1	0	first.t
+		EOF
+	)
+'
+
+test_expect_success '--empty=drop drops empty replayed commits' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		touch base remove-me &&
+		git add . &&
+		git commit -m "base" &&
+		git rm remove-me &&
+		git commit -m "remove" &&
+		touch reintroduce remove-me &&
+		git add . &&
+		git commit -m "reintroduce" &&
+
+		git rm remove-me &&
+		git history fixup --empty=drop HEAD~2 &&
+
+		expect_changes <<-\EOF
+		reintroduce
+		0	0	reintroduce
+		0	0	remove-me
+		base
+		0	0	base
+		EOF
+	)
+'
+
+test_expect_success '--empty=keep keeps commit when fixup target becomes empty' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		test_commit first &&
+		test_commit second &&
+		test_commit third &&
+
+		git rm second.t &&
+		git history fixup --empty=keep HEAD~ &&
+
+		expect_changes <<-\EOF
+		third
+		1	0	third.t
+		second
+		first
+		1	0	first.t
+		EOF
+	)
+'
+
+test_expect_success '--empty=keep keeps commit when replayed commit becomes empty' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		touch base remove-me &&
+		git add . &&
+		git commit -m "base" &&
+		git rm remove-me &&
+		git commit -m "remove" &&
+		touch reintroduce remove-me &&
+		git add . &&
+		git commit -m "reintroduce" &&
+
+		git rm remove-me &&
+		git history fixup --empty=keep HEAD~2 &&
+
+		expect_changes <<-\EOF
+		reintroduce
+		0	0	reintroduce
+		0	0	remove-me
+		remove
+		base
+		0	0	base
+		EOF
+	)
+'
+
+test_expect_success '--empty=abort errors out when fixup target becomes empty' '
+	test_when_finished "rm -rf repo" &&
+	git init repo &&
+	(
+		cd repo &&
+
+		test_commit first &&
+		test_commit second &&
+
+		git rm first.t &&
+		test_must_fail git history fixup --empty=abort HEAD~ 2>err &&
+		test_grep "fixup makes commit.*empty" err
+	)
+'
+
+test_expect_success '--empty=abort errors out when a descendant becomes empty during replay' '
+	test_when_finished "rm -rf repo" &&
+	git init repo --initial-branch=main &&
+	(
+		cd repo &&
+
+		touch base remove-me &&
+		git add . &&
+		git commit -m "base" &&
+		git rm remove-me &&
+		git commit -m "remove" &&
+		touch reintroduce remove-me &&
+		git add . &&
+		git commit -m "reintroduce" &&
+
+		git rm remove-me &&
+		test_must_fail git history fixup --empty=abort HEAD~2 2>err &&
+		test_grep "became empty after replay" err
+	)
+'
+
+test_done