Merge branch 'td/describe-tag-iteration' into jch

'git describe' has been taught to pass the 'refs/tags/' prefix down to
the ref iterator when '--all' is not requested, avoiding unnecessary
iteration over non-tag refs.

* td/describe-tag-iteration:
  describe: limit default ref iteration to tags

builtin/describe.c

@@ -752,6 +752,9 @@ 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