describe: limit default ref iteration to tags

Without --all, git describe ignores refs outside refs/tags/. Commit
8a5a1884e9 (Avoid accessing non-tag refs in git-describe unless --all is
requested, 2008-02-24) moved this check ahead of object lookup. That
avoided loading objects for irrelevant refs, but the backend still has
to yield every ref before get_name() can reject it.

Pass refs/tags/ to the iterator so the backend can avoid visiting those
refs in the first place.

The new perf test creates 10,000 unrelated packed refs. It measures:

    git describe --exact-match HEAD

The runtime drops from 0.03(0.01+0.01) to 0.02(0.00+0.00). In a
repository with 120,532 refs but only 330 tags, the same command went
from 171.7 ms to 9.9 ms.

Signed-off-by: Tamir Duberstein <tamird@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/describe.c

@@ -740,6 +740,9 @@ int cmd_describe(int argc,
 		return ret;
 	}
 
+	if (!all)
+		for_each_ref_opts.prefix = "refs/tags/";
+
 	hashmap_init(&names, commit_name_neq, NULL, 0);
 	refs_for_each_ref_ext(get_main_ref_store(the_repository),
 			      get_name, NULL, &for_each_ref_opts);

t/perf/p6100-describe.sh

@@ -27,4 +27,16 @@ test_perf 'describe HEAD with one tag' '
 	git describe --match=new HEAD
 '
 
+test_expect_success 'set up many unrelated refs' '
+	ref_count=10000 &&
+	git tag -m tip tip HEAD &&
+	test_seq -f "create refs/heads/describe-perf/%05d HEAD" $ref_count |
+	git update-ref --stdin &&
+	git pack-refs --all
+'
+
+test_perf 'describe exact tag with many unrelated refs' '
+	git describe --exact-match HEAD
+'
+
 test_done