commit: use generation numbers for in_merge_bases()

The containment algorithm for 'git branch --contains' is different
from that for 'git tag --contains' in that it uses is_descendant_of()
instead of contains_tag_algo(). The expensive portion of the branch
algorithm is computing merge bases.

When a commit-graph file exists with generation numbers computed,
we can avoid this merge-base calculation when the target commit has
a larger generation number than the initial commits.

Performance tests were run on a copy of the Linux repository where
HEAD is contained in v4.13 but no earlier tag. Also, all tags were
copied to branches and 'git branch --contains' was tested:

Before: 60.0s
After:   0.4s
Rel %: -99.3%

Reported-by: Jeff King <peff@peff.net>
Signed-off-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit.c

@@ -1056,12 +1056,19 @@ int in_merge_bases_many(struct commit *commit, int nr_reference, struct commit *
 {
 	struct commit_list *bases;
 	int ret = 0, i;
+	uint32_t min_generation = GENERATION_NUMBER_INFINITY;
 
 	if (parse_commit(commit))
 		return ret;
-	for (i = 0; i < nr_reference; i++)
+	for (i = 0; i < nr_reference; i++) {
 		if (parse_commit(reference[i]))
 			return ret;
+		if (reference[i]->generation < min_generation)
+			min_generation = reference[i]->generation;
+	}
+
+	if (commit->generation > min_generation)
+		return ret;
 
 	bases = paint_down_to_common(commit, nr_reference, reference);
 	if (commit->object.flags & PARENT2)