ref-filter: memoize --contains with generations

git branch and git for-each-ref run a separate reachability walk for
each ref considered by --contains and --no-contains. Refs with shared
history therefore traverse the same commits repeatedly.

git tag instead uses a depth-first walk that caches results across
refs. That walk can perform poorly without generation numbers: a
negative check may walk to the root instead of stopping at a nearby
divergence. Generation numbers let it stop below the oldest target.

Use the memoized walk for all ref-filter callers when generation
numbers are available. Keep git tag on its existing path without
generations. Caching still helps when many tags share deep history:
ffc4b8012d (tag: speed up --contains calculation, 2011-06-11) reduced
git tag --contains HEAD~200 in linux-2.6 from 15.417 to 5.329 seconds.

The new shared-history perf test improves from 0.72 to 0.03 seconds. In
a repository with 62,174 remote-tracking refs, running:

    git branch -r --contains c78ae85f3ce7e

improves from 104.365 seconds to 468 milliseconds.

Suggested-by: Jeff King <peff@peff.net>
Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

commit-reach.c

diff --git a/commit-reach.c b/commit-reach.c
index 58553df..349ca68 100644
--- a/commit-reach.c
+++ b/commit-reach.c
@@ -854,7 +854,8 @@ static enum contains_result contains_tag_algo(struct commit *candidate,
 int commit_contains(struct ref_filter *filter, struct commit *commit,
 		    struct commit_list *list, struct contains_cache *cache)
 {
-	if (filter->with_commit_tag_algo)
+	if (filter->with_commit_tag_algo ||
+	    generation_numbers_enabled(the_repository))
 		return contains_tag_algo(commit, list, cache) == CONTAINS_YES;
 	return repo_is_descendant_of(the_repository, commit, list);
 }

t/perf/p1500-graph-walks.sh

diff --git a/t/perf/p1500-graph-walks.sh b/t/perf/p1500-graph-walks.sh
index 5b23ce5..d167b4f 100755
--- a/t/perf/p1500-graph-walks.sh
+++ b/t/perf/p1500-graph-walks.sh
@@ -32,7 +32,16 @@
 		echo "X:$line" >>test-tool-tags || return 1
 	done &&
 
-	commit=$(git commit-tree $(git rev-parse HEAD^{tree})) &&
+	git rev-list --first-parent --max-count=8192 HEAD >contains-commits &&
+	test_file_not_empty contains-commits &&
+	git update-ref refs/contains-perf-base "$(tail -n 1 contains-commits)" &&
+	awk "{
+		printf \"update refs/contains-perf/%04d %s\\n\", NR, \$1
+	}" contains-commits |
+		git update-ref --stdin &&
+	git pack-refs --include "refs/contains-perf/*" &&
+
+	commit=$(git commit-tree HEAD^{tree}) &&
 	git update-ref refs/heads/disjoint-base $commit &&
 
 	git commit-graph write --reachable
@@ -62,6 +71,23 @@
 	xargs git tag --merged=HEAD <tags
 '
 
+test_perf 'contains: git for-each-ref' '
+	git for-each-ref --contains=refs/contains-perf-base --stdin <refs
+'
+
+test_perf 'contains: git branch' '
+	xargs git branch --contains=refs/contains-perf-base <branches
+'
+
+test_perf 'contains: git tag' '
+	xargs git tag --contains=refs/contains-perf-base <tags
+'
+
+test_perf 'contains: synthetic shared history' '
+	git for-each-ref --contains=refs/contains-perf-base \
+		refs/contains-perf/ >/dev/null
+'
+
 test_perf 'is-base check: test-tool reach (refs)' '
 	test-tool reach get_branch_base_for_tip <test-tool-refs
 '