pack-objects: support filters with bitmaps

Just as rev-list recently learned to combine filters and bitmaps, let's
do the same for pack-objects. The infrastructure is all there; we just
need to pass along our filter options, and the pack-bitmap code will
decide to use bitmaps or not.

This unsurprisingly makes things faster for partial clones of large
repositories (here we're cloning linux.git):

  Test                               HEAD^               HEAD
  ------------------------------------------------------------------------------
  5310.11: simulated partial clone   38.94(37.28+5.87)   11.06(11.27+4.07) -71.6%

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/pack-objects.c

@@ -3040,7 +3040,7 @@ static int pack_options_allow_reuse(void)
 
 static int get_object_list_from_bitmap(struct rev_info *revs)
 {
-	if (!(bitmap_git = prepare_bitmap_walk(revs, NULL)))
+	if (!(bitmap_git = prepare_bitmap_walk(revs, &filter_options)))
 		return -1;
 
 	if (pack_options_allow_reuse() &&
@@ -3419,7 +3419,6 @@ int cmd_pack_objects(int argc, const char **argv, const char *prefix)
 	if (filter_options.choice) {
 		if (!pack_to_stdout)
 			die(_("cannot use --filter without --stdout"));
-		use_bitmap_index = 0;
 	}
 
 	/*

t/perf/p5310-pack-bitmaps.sh

@@ -57,6 +57,10 @@ test_perf 'rev-list count with blob:limit=1k' '
 		--filter=blob:limit=1k >/dev/null
 '
 
+test_perf 'simulated partial clone' '
+	git pack-objects --stdout --all --filter=blob:none </dev/null >/dev/null
+'
+
 test_expect_success 'create partial bitmap state' '
 	# pick a commit to represent the repo tip in the past
 	cutoff=$(git rev-list HEAD~100 -1) &&

t/t5310-pack-bitmaps.sh

@@ -107,6 +107,20 @@ test_expect_success 'clone from bitmapped repository' '
 	test_cmp expect actual
 '
 
+test_expect_success 'partial clone from bitmapped repository' '
+	test_config uploadpack.allowfilter true &&
+	git clone --no-local --bare --filter=blob:none . partial-clone.git &&
+	(
+		cd partial-clone.git &&
+		pack=$(echo objects/pack/*.pack) &&
+		git verify-pack -v "$pack" >have &&
+		awk "/blob/ { print \$1 }" <have >blobs &&
+		# we expect this single blob because of the direct ref
+		git rev-parse refs/tags/tagged-blob >expect &&
+		test_cmp expect blobs
+	)
+'
+
 test_expect_success 'setup further non-bitmapped commits' '
 	test_commit_bulk --id=further 10
 '