merge-base --octopus to mimic show-branch --merge-base

While show-branch --merge-base does not support more than MAX_REVS
revs, git supports more with a different algorithm
(v1.6.0-rc0~51^2~13, Introduce get_octopus_merge_bases() in commit.c,
2008-06-27).  Expose that functionality.

This should help scripts to catch up with builtin merge in supporting
dodecapus.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-merge-base.txt

@@ -8,7 +8,7 @@ git-merge-base - Find as good common ancestors as possible for a merge
 
 SYNOPSIS
 --------
-'git merge-base' [-a|--all] <commit> <commit>...
+'git merge-base' [-a|--all] [--octopus] <commit> <commit>...
 
 DESCRIPTION
 -----------
@@ -20,12 +20,12 @@ that does not have any better common ancestor is a 'best common
 ancestor', i.e. a 'merge base'.  Note that there can be more than one
 merge base for a pair of commits.
 
-Among the two commits to compute the merge base from, one is specified by
-the first commit argument on the command line; the other commit is a
-(possibly hypothetical) commit that is a merge across all the remaining
-commits on the command line.  As the most common special case, specifying only
-two commits on the command line means computing the merge base between
-the given two commits.
+Unless `--octopus` is given, among the two commits to compute the merge
+base from, one is specified by the first commit argument on the command
+line; the other commit is a (possibly hypothetical) commit that is a merge
+across all the remaining commits on the command line.  As the most common
+special case, specifying only two commits on the command line means
+computing the merge base between the given two commits.
 
 As a consequence, the 'merge base' is not necessarily contained in each of the
 commit arguments if more than two commits are specified. This is different
@@ -37,6 +37,11 @@ OPTIONS
 --all::
 	Output all merge bases for the commits, instead of just one.
 
+--octopus::
+	Compute the best common ancestors of all supplied commits,
+	in preparation for an n-way merge.  This mimics the behavior
+	of 'git show-branch --merge-base'.
+
 DISCUSSION
 ----------
 

builtin/merge-base.c

@@ -23,7 +23,7 @@ static int show_merge_base(struct commit **rev, int rev_nr, int show_all)
 }
 
 static const char * const merge_base_usage[] = {
-	"git merge-base [-a|--all] <commit> <commit>...",
+	"git merge-base [-a|--all] [--octopus] <commit> <commit>...",
 	NULL
 };
 
@@ -41,21 +41,50 @@ static struct commit *get_commit_reference(const char *arg)
 	return r;
 }
 
+static int show_octopus_merge_bases(int count, const char **args, int show_all)
+{
+	struct commit_list *revs = NULL;
+	struct commit_list *result;
+	int i;
+
+	for (i = count - 1; i >= 0; i++)
+		commit_list_insert(get_commit_reference(args[i]), &revs);
+	result = get_octopus_merge_bases(revs);
+
+	if (!result)
+		return 1;
+
+	while (result) {
+		printf("%s\n", sha1_to_hex(result->item->object.sha1));
+		if (!show_all)
+			return 0;
+		result = result->next;
+	}
+
+	return 0;
+}
+
 int cmd_merge_base(int argc, const char **argv, const char *prefix)
 {
 	struct commit **rev;
 	int rev_nr = 0;
 	int show_all = 0;
+	int octopus = 0;
 
 	struct option options[] = {
-		OPT_BOOLEAN('a', "all", &show_all, "outputs all common ancestors"),
+		OPT_BOOLEAN('a', "all", &show_all, "output all common ancestors"),
+		OPT_BOOLEAN(0, "octopus", &octopus, "find ancestors for a single n-way merge"),
 		OPT_END()
 	};
 
 	git_config(git_default_config, NULL);
 	argc = parse_options(argc, argv, prefix, options, merge_base_usage, 0);
-	if (argc < 2)
+	if (!octopus && argc < 2)
 		usage_with_options(merge_base_usage, options);
+
+	if (octopus)
+		return show_octopus_merge_bases(argc, argv, show_all);
+
 	rev = xmalloc(argc * sizeof(*rev));
 	while (argc-- > 0)
 		rev[rev_nr++] = get_commit_reference(*argv++);

t/t6010-merge-base.sh

@@ -164,9 +164,11 @@ test_expect_success 'merge-base A B C' '
 	git rev-parse --verify MMR >expected.sb &&
 
 	git merge-base --all MMA MMB MMC >actual &&
+	git merge-base --all --octopus MMA MMB MMC >actual.common &&
 	git show-branch --merge-base MMA MMB MMC >actual.sb &&
 
 	test_cmp expected actual &&
+	test_cmp expected.sb actual.common &&
 	test_cmp expected.sb actual.sb
 '