[PATCH v3] merge-tree.c: add --merge-base=<commit> option

From: Kyle Zhao <kylezhao@tencent.com>

This patch will give our callers more flexibility to use `git merge-tree`, such as:

    git merge-tree --write-tree --merge-base=branch^ HEAD branch

This does a merge of HEAD and branch, but uses branch^ as the merge-base.

And the reason why using an option flag instead of a positional argument is to allow additional commits passed to merge-tree to be handled via an octopus merge in the future.

Signed-off-by: Kyle Zhao <kylezhao@tencent.com>
---
    merge-tree: allow specifying a base commit when --write-tree is passed
    
    Thanks for Elijah's work. I'm very excited that merge-ort is integrated
    into the git merge-tree, which means that we can use merge-ort in bare
    repositories to optimize merge performance.
    
    In this patch, I introduce a new --merge-base=<commit> option to allow
    callers to specify a merge-base for the merge. This may allow users to
    implement git cherry-pick and git rebase in bare repositories with git
    merge-tree cmd.
    
    Changes since v1:
    
     * Changed merge_incore_recursive() to merge_incore_nonrecursive() when
       merge-base is specified.
     * Fixed c style problem.
     * Moved commit lookup/die logic out to the parsing logic in
       cmd_merge_tree().
     * use test_commit for test
    
    Changes since v2:
    
     * commit message
     * Rebased on top of en/merge-tree-sequence.
     * Set opt.ancestor to o->merge_base. Because opt.ancestor is a *char.
       To make it easier to pass parameters, I moved
       lookup_commit_reference_by_name() to real_ merge() again.
     * Added test comment.

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-1397%2Fkeyu98%2Fkz%2Fmerge-tree-option-merge-base-v3
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-1397/keyu98/kz/merge-tree-option-merge-base-v3
Pull-Request: https://github.com/gitgitgadget/git/pull/1397

Range-diff vs v2:

 1:  ab4e5d5ad08 ! 1:  554cbde49e8 merge-tree.c: add --merge-base=<commit> option
     @@ Commit message
      
              git merge-tree --write-tree --merge-base=branch^ HEAD branch
      
     -    It would cherry-pick the commit at the tip of the branch on top of the
     -    current commit even if the repository is bare.
     +    This does a merge of HEAD and branch, but uses branch^ as the merge-base.
     +
     +    And the reason why using an option flag instead of a positional argument
     +    is to allow additional commits passed to merge-tree to be handled via an
     +    octopus merge in the future.
      
          Signed-off-by: Kyle Zhao <kylezhao@tencent.com>
      
     @@ builtin/merge-tree.c
       #include "merge-ort.h"
       #include "object-store.h"
      @@ builtin/merge-tree.c: struct merge_tree_options {
     - 	int allow_unrelated_histories;
       	int show_messages;
       	int name_only;
     -+	const struct commit *base_commit;
     + 	int use_stdin;
     ++	const char *merge_base;
       };
       
       static int real_merge(struct merge_tree_options *o,
     @@ builtin/merge-tree.c: static int real_merge(struct merge_tree_options *o,
      -	if (!merge_bases && !o->allow_unrelated_histories)
      -		die(_("refusing to merge unrelated histories"));
      -	merge_bases = reverse_commit_list(merge_bases);
     -+	if (o->base_commit) {
     ++	if (o->merge_base) {
     ++		struct commit *base_commit;
      +		struct tree *base_tree, *parent1_tree, *parent2_tree;
      +
     -+		opt.ancestor = "specified merge base";
     -+		base_tree = get_commit_tree(o->base_commit);
     ++		base_commit = lookup_commit_reference_by_name(o->merge_base);
     ++		if (!base_commit)
     ++			die(_("could not lookup commit %s"), o->merge_base);
     ++
     ++		opt.ancestor = o->merge_base;
     ++		base_tree = get_commit_tree(base_commit);
      +		parent1_tree = get_commit_tree(parent1);
      +		parent2_tree = get_commit_tree(parent2);
      +		merge_incore_nonrecursive(&opt, base_tree, parent1_tree, parent2_tree, &result);
     @@ builtin/merge-tree.c: static int real_merge(struct merge_tree_options *o,
       	if (result.clean < 0)
       		die(_("failure to merge"));
       
     -@@ builtin/merge-tree.c: static int real_merge(struct merge_tree_options *o,
     - 
     - int cmd_merge_tree(int argc, const char **argv, const char *prefix)
     - {
     -+	const char *merge_base = NULL;
     - 	struct merge_tree_options o = { .show_messages = -1 };
     - 	int expected_remaining_argc;
     - 	int original_argc;
      @@ builtin/merge-tree.c: int cmd_merge_tree(int argc, const char **argv, const char *prefix)
     - 			   &o.allow_unrelated_histories,
     - 			   N_("allow merging unrelated histories"),
     + 			   &o.use_stdin,
     + 			   N_("perform multiple merges, one per line of input"),
       			   PARSE_OPT_NONEG),
      +		OPT_STRING(0, "merge-base",
     -+			 &merge_base,
     -+			 N_("commit"),
     -+			 N_("specify a merge-base for the merge")),
     ++			   &o.merge_base,
     ++			   N_("commit"),
     ++			   N_("specify a merge-base for the merge")),
       		OPT_END()
       	};
       
     -@@ builtin/merge-tree.c: int cmd_merge_tree(int argc, const char **argv, const char *prefix)
     - 		usage_with_options(merge_tree_usage, mt_options);
     - 
     - 	/* Do the relevant type of merge */
     --	if (o.mode == MODE_REAL)
     -+	if (o.mode == MODE_REAL) {
     -+		if (merge_base) {
     -+			o.base_commit = lookup_commit_reference_by_name(merge_base);
     -+			if (!o.base_commit)
     -+				die(_("could not lookup commit %s"), merge_base);
     -+		}
     - 		return real_merge(&o, argv[0], argv[1], prefix);
     -+	}
     - 	else
     - 		return trivial_merge(argv[0], argv[1], argv[2]);
     - }
      
       ## t/t4301-merge-tree-write-tree.sh ##
     -@@ t/t4301-merge-tree-write-tree.sh: test_expect_success SANITY 'merge-ort fails gracefully in a read-only repository
     - 	test_must_fail git -C read-only merge-tree side1 side2
     +@@ t/t4301-merge-tree-write-tree.sh: test_expect_success '--stdin with both a successful and a conflicted merge' '
     + 	test_cmp expect actual
       '
       
     ++# specify merge-base as parent of branch2
     ++# git merge-tree --write-tree --merge-base=c2 c1 c3
     ++#   Commit c1: add file1
     ++#   Commit c2: add file2 after c1
     ++#   Commit c3: add file3 after c2
     ++#   Expected: add file3, and file2 does NOT appear
     ++
      +test_expect_success 'specify merge-base as parent of branch2' '
      +	# Setup
      +	git init base-b2-p && (

 Documentation/git-merge-tree.txt |  4 ++++
 builtin/merge-tree.c             | 39 ++++++++++++++++++++++++--------
 t/t4301-merge-tree-write-tree.sh | 30 ++++++++++++++++++++++++
 3 files changed, 64 insertions(+), 9 deletions(-)