merge-base --independent to print reduced parent list in a merge

While show-branch --independent does not support more than MAX_REVS
revs, git internally supports more with a different algorithm.
Expose that functionality as "git merge-base --independent".

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>

1 files changed, 15 insertions, 6 deletions

diff --git a/builtin/merge-base.c b/builtin/merge-base.c
index c30128659f..96dd160731 100644
--- a/builtin/merge-base.c
+++ b/builtin/merge-base.c
@@ -24,6 +24,7 @@ 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] [--octopus] <commit> <commit>...",
+	"git merge-base --independent <commit>...",
 	NULL
 };
 
@@ -41,15 +42,19 @@ 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)
+static int handle_octopus(int count, const char **args, int reduce, int show_all)
 {
 	struct commit_list *revs = NULL;
 	struct commit_list *result;
 	int i;
 
-	for (i = count - 1; i >= 0; i++)
+	if (reduce)
+		show_all = 1;
+
+	for (i = count - 1; i >= 0; i--)
 		commit_list_insert(get_commit_reference(args[i]), &revs);
-	result = get_octopus_merge_bases(revs);
+
+	result = reduce ? reduce_heads(revs) : get_octopus_merge_bases(revs);
 
 	if (!result)
 		return 1;
@@ -70,20 +75,24 @@ int cmd_merge_base(int argc, const char **argv, const char *prefix)
 	int rev_nr = 0;
 	int show_all = 0;
 	int octopus = 0;
+	int reduce = 0;
 
 	struct option options[] = {
 		OPT_BOOLEAN('a', "all", &show_all, "output all common ancestors"),
 		OPT_BOOLEAN(0, "octopus", &octopus, "find ancestors for a single n-way merge"),
+		OPT_BOOLEAN(0, "independent", &reduce, "list revs not reachable from others"),
 		OPT_END()
 	};
 
 	git_config(git_default_config, NULL);
 	argc = parse_options(argc, argv, prefix, options, merge_base_usage, 0);
-	if (!octopus && argc < 2)
+	if (!octopus && !reduce && argc < 2)
 		usage_with_options(merge_base_usage, options);
+	if (reduce && (show_all || octopus))
+		die("--independent cannot be used with other options");
 
-	if (octopus)
-		return show_octopus_merge_bases(argc, argv, show_all);
+	if (octopus || reduce)
+		return handle_octopus(argc, argv, reduce, show_all);
 
 	rev = xmalloc(argc * sizeof(*rev));
 	while (argc-- > 0)