Merge branch 'js/gc-with-stale-symref' into maint

"git gc" used to barf when a symbolic ref has gone dangling
(e.g. the branch that used to be your upstream's default when you
cloned from it is now gone, and you did "fetch --prune").

* js/gc-with-stale-symref:
  pack-objects: do not get distracted by broken symrefs
  gc: demonstrate failure with stale remote HEAD

reachable.c

@@ -25,9 +25,15 @@ static void update_progress(struct connectivity_progress *cp)
 static int add_one_ref(const char *path, const struct object_id *oid,
 		       int flag, void *cb_data)
 {
-	struct object *object = parse_object_or_die(oid->hash, path);
 	struct rev_info *revs = (struct rev_info *)cb_data;
+	struct object *object;
 
+	if ((flag & REF_ISSYMREF) && (flag & REF_ISBROKEN)) {
+		warning("symbolic ref is dangling: %s", path);
+		return 0;
+	}
+
+	object = parse_object_or_die(oid->hash, path);
 	add_pending_object(revs, object, "");
 
 	return 0;

t/t6500-gc.sh

@@ -30,4 +30,17 @@ test_expect_success 'gc -h with invalid configuration' '
 	test_i18ngrep "[Uu]sage" broken/usage
 '
 
+test_expect_success 'gc is not aborted due to a stale symref' '
+	git init remote &&
+	(
+		cd remote &&
+		test_commit initial &&
+		git clone . ../client &&
+		git branch -m develop &&
+		cd ../client &&
+		git fetch --prune &&
+		git gc
+	)
+'
+
 test_done