Just detect exit status rather than check ref

Feedback:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/13416#note_37193731

So we just try a cheaper way to detect it if it works or not

app/models/repository.rb

@@ -1024,7 +1024,7 @@ class Repository
     message, status = run_git(args)
     # Make sure ref was created, and raise Rugged::ReferenceError when not
-    raise Rugged::ReferenceError, message unless ref_exists?(target_ref)
+    raise Rugged::ReferenceError, message if status != 0
   end
   def create_ref(ref, ref_path)