in_merge_bases(): omit unnecessary redundant common ancestor reduction

The function get_merge_bases() needs to postprocess the result from
merge_bases_many() in order to make sure none of the commit is a
true ancestor of another commit, which is expensive.  However, when
checking if a commit is an ancestor of another commit, we only need
to see if the commit is a common ancestor between the two, and do
not have to care if other common ancestors merge_bases_many() finds
are true merge bases or an ancestor of another merge base.

Signed-off-by: Thomas Rast <trast@student.ethz.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -780,7 +780,10 @@ int in_merge_bases(struct commit *commit, struct commit *reference)
 	struct commit_list *bases, *b;
 	int ret = 0;
 
-	bases = get_merge_bases(commit, reference, 1);
+	bases = merge_bases_many(commit, 1, &reference);
+	clear_commit_marks(commit, all_flags);
+	clear_commit_marks(reference, all_flags);
+
 	for (b = bases; b; b = b->next) {
 		if (!hashcmp(commit->object.sha1, b->item->object.sha1)) {
 			ret = 1;