Re: [PATCH v6 05/11] builtin/history: implement "reword" subcommand

> +	struct setup_revision_opt revision_opts = {
> +		.assume_dashdash = 1,
> +	};
> +	struct strvec revisions = STRVEC_INIT;
> +	struct commit *child;
> +	struct rev_info rev = { 0 };
> +	int ret;
> +
> +	repo_init_revisions(repo, &rev, NULL);
> +	strvec_push(&revisions, "");
> +	strvec_push(&revisions, oid_to_hex(&new_commit->object.oid));
> +	if (old_commit)
> +		strvec_pushf(&revisions, "^%s", oid_to_hex(&old_commit->object.oid));
> +
> +	setup_revisions_from_strvec(&revisions, &rev, &revision_opts);
> +	if (revisions.nr != 1 || prepare_revision_walk(&rev)) {

> +static int apply_commits(struct repository *repo,
> +			 const struct strvec *commits,
> +			 struct commit *onto,
> +			 struct commit *orig_head,
> +			 const char *action)
> +{
> +	struct reset_head_opts reset_opts = { 0 };
> +	struct strbuf buf = STRBUF_INIT;
> +	int ret;
> +
> +	for (size_t i = 0; i < commits->nr; i++) {
> +		struct object_id commit_id;
> +		struct commit *commit;
> +		const char *end;
> +
> +		if (parse_oid_hex_algop(commits->v[i], &commit_id, &end,
> +					repo->hash_algo)) {
> +			ret = error(_("invalid object ID: %s"), commits->v[i]);
> +			goto out;
> +		}
> +		commit = lookup_commit(repo, &commit_id);
> +		if (!commit || repo_parse_commit(repo, commit)) {
> +			ret = error(_("failed to look up commit: %s"), oid_to_hex(&commit_id));
> +			goto out;
> +		}

> +		if (!onto) {
> +			onto = commit;
> +		} else {
> +			struct tree *tree = repo_get_commit_tree(repo, commit);
> +			onto = replay_create_commit(repo, tree, commit, onto);
> +			if (!onto)
> +				break;

> +		}
> +	}
> +
> +	reset_opts.oid = &onto->object.oid;
> +	strbuf_addf(&buf, "%s: switch to rewritten %s", action, oid_to_hex(reset_opts.oid));

> +static int fill_commit_message(struct repository *repo,
> +			       const struct object_id *old_tree,
> +			       const struct object_id *new_tree,
> +			       const char *default_message,
> +			       const char *action,
> +			       struct strbuf *out)
> +{
> +	const char *path = git_path_commit_editmsg();
> +	const char *hint =
> +		_("Please enter the commit message for the %s changes."

> +		  " Lines starting\nwith '%s' will be ignored.\n");
> +	struct wt_status s;
> +
> +	strbuf_addstr(out, default_message);
> +	strbuf_addch(out, '\n');
> +	strbuf_commented_addf(out, comment_line_str, hint, action, comment_line_str);
> +	write_file_buf(path, out->buf, out->len);
> +
> +	wt_status_prepare(repo, &s);
> +	FREE_AND_NULL(s.branch);
> +	s.ahead_behind_flags = AHEAD_BEHIND_QUICK;
> +	s.commit_template = 1;
> +	s.colopts = 0;
> +	s.display_comment_prefix = 1;
> +	s.hints = 0;
> +	s.use_color = 0;
> +	s.whence = FROM_COMMIT;
> +	s.committable = 1;

> +	s.fp = fopen(git_path_commit_editmsg(), "a");
> +	if (!s.fp)
> +		return error_errno(_("could not open '%s'"), git_path_commit_editmsg());
> +
> +	wt_status_collect_changes_trees(&s, old_tree, new_tree);
> +	wt_status_print(&s);
> +	wt_status_collect_free_buffers(&s);
> +	string_list_clear_func(&s.change, change_data_free);
> +
> +	strbuf_reset(out);
> +	if (launch_editor(path, out, NULL)) {
> +		fprintf(stderr, _("Please supply the message using the -m option.\n"));

> +static int cmd_history_reword(int argc,
> +			      const char **argv,
> +			      const char *prefix,
> +			      struct repository *repo)
> +{
> +	const char * const usage[] = {
> +		GIT_HISTORY_REWORD_USAGE,
> +		NULL,
> +	};
> +	struct option options[] = {
> +		OPT_END(),
> +	};
> +	struct strbuf final_message = STRBUF_INIT;
> +	struct commit *original_commit, *parent, *head;
> +	struct strvec commits = STRVEC_INIT;
> +	struct object_id parent_tree_oid, original_commit_tree_oid;
> +	struct object_id rewritten_commit;
> +	struct commit_list *from_list = NULL;
> +	const char *original_message, *original_body, *ptr;
> +	char *original_author = NULL;
> +	size_t len;
> +	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);
> +
> +	original_commit = lookup_commit_reference_by_name(argv[0]);
> +	if (!original_commit) {
> +		ret = error(_("commit to be reworded cannot be found: %s"), argv[0]);
> +		goto out;
> +	}
> +	original_commit_tree_oid = repo_get_commit_tree(repo, original_commit)->object.oid;

> +
> +	parent = original_commit->parents ? original_commit->parents->item : NULL;
> +	if (parent) {
> +		if (repo_parse_commit(repo, parent)) {
> +			ret = error(_("unable to parse commit %s"),
> +				    oid_to_hex(&parent->object.oid));
> +			goto out;
> +		}
> +		parent_tree_oid = repo_get_commit_tree(repo, parent)->object.oid;
> +	} else {
> +		oidcpy(&parent_tree_oid, repo->hash_algo->empty_tree);
> +	}
> +
> +	head = lookup_commit_reference_by_name("HEAD");
> +	if (!head) {
> +		ret = error(_("could not resolve HEAD to a commit"));
> +		goto out;
> +	}
> +
> +	commit_list_append(original_commit, &from_list);
> +	if (!repo_is_descendant_of(repo, head, from_list)) {
> +		ret = error (_("split commit must be reachable from current HEAD commit"));

> +		goto out;
> +	}
> +
> +	/*
> +	 * Collect the list of commits that we'll have to reapply now already.
> +	 * This ensures that we'll abort early on in case the range of commits
> +	 * contains merges, which we do not yet handle.
> +	 */

> diff --git a/t/t3450-history.sh b/t/t3450-history.sh
> index 417c343d43b..f513463b92b 100755
> --- a/t/t3450-history.sh
> +++ b/t/t3450-history.sh
> @@ -5,13 +5,13 @@ test_description='tests for git-history command'
>   . ./test-lib.sh
>   
>   test_expect_success 'does nothing without any arguments' '
> -	git history >out 2>&1 &&
> -	test_must_be_empty out
> +	test_must_fail git history 2>err &&
> +	test_grep "need a subcommand" err
>   '
>   
>   test_expect_success 'raises an error with unknown argument' '
>   	test_must_fail git history garbage 2>err &&
> -	test_grep "unrecognized argument: garbage" err
> +	test_grep "unknown subcommand: .garbage." err
>   '
>   
>   test_done

> diff --git a/t/t3451-history-reword.sh b/t/t3451-history-reword.sh
> new file mode 100755
> index 00000000000..09dbc463c59
> --- /dev/null
> +++ b/t/t3451-history-reword.sh
> @@ -0,0 +1,237 @@
> +#!/bin/sh
> +
> +test_description='tests for git-history reword subcommand'
> +
> +. ./test-lib.sh
> +
> +reword_with_message () {
> +	cat >message &&
> +	write_script fake-editor.sh <<-EOF &&
> +	cp "$(pwd)/message" "\$1"

> +	test_when_finished "rm -rf repo" &&
> +	git init repo &&
> +	(
> +		cd repo &&
> +		test_commit first &&
> +
> +		write_script fake-editor.sh <<-\EOF &&
> +		cp "$1" . &&
> +		printf "\namend a comment\n" >>"$1"
> +		EOF
> +		test_set_editor "$(pwd)"/fake-editor.sh &&
> +		git history reword HEAD &&
> +
> +		cat >expect <<-EOF &&
> +		first
> +
> +		# Please enter the commit message for the reworded changes. Lines starting
> +		# with ${SQ}#${SQ} will be ignored.
> +		# Changes to be committed:
> +		#	new file:   first.t
> +		#
> +		EOF
> +		test_cmp expect COMMIT_EDITMSG &&
> +
> +		cat >expect <<-EOF &&
> +		first
> +
> +		amend a comment
> +
> +		EOF
> +		git log --format=%B >actual &&
> +		test_cmp expect actual

> +	)
> +'
> +
> +# For now, git-history(1) does not yet execute any hooks. This is subject to
> +# change in the future, and if it does this test here is expected to start
> +# failing. In other words, this test is not an endorsement of the current
> +# status quo.
> +test_expect_success 'hooks are not executed for rewritten commits' '
> +	test_when_finished "rm -rf repo" &&
> +	git init repo &&
> +	(
> +		cd repo &&
> +		test_commit first &&
> +		test_commit second &&
> +		test_commit third &&
> +
> +		write_script .git/hooks/prepare-commit-msg <<-EOF &&
> +		touch "$(pwd)/hooks.log

> +		EOF
> +		write_script .git/hooks/post-commit <<-EOF &&
> +		touch "$(pwd)/hooks.log
> +		EOF
> +		write_script .git/hooks/post-rewrite <<-EOF &&
> +		touch "$(pwd)/hooks.log
> +		EOF