Always return MR diff_refs if importing

This change fixes a bug that prevents projects being
imported when there are diff notes present in merge requests.

Solves https://gitlab.com/gitlab-org/gitlab-ce/issues/63955

app/models/merge_request.rb

@@ -588,6 +588,8 @@ class MergeRequest < ApplicationRecord
   end
 
   def diff_refs
+    return merge_request_diff.diff_refs if importing?
+
     persisted? ? merge_request_diff.diff_refs : repository_diff_refs
   end