pack-refs: fix git_path() usage.

Signed-off-by: Junio C Hamano <junkio@cox.net>

builtin-pack-refs.c

@@ -56,7 +56,7 @@ static void prune_ref(struct ref_to_prune *r)
 	struct ref_lock *lock = lock_ref_sha1(r->name + 5, r->sha1, 1);
 
 	if (lock) {
-		unlink(git_path(r->name));
+		unlink(git_path("%s", r->name));
 		unlock_ref(lock);
 	}
 }