Make sure merge request is fetched before collecting base sha in merge request diff

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/models/merge_request_diff.rb

@@ -41,6 +41,7 @@ class MergeRequestDiff < ActiveRecord::Base
   end
 
   def ensure_commits_sha
+    merge_request.fetch_ref
     self.start_commit_sha ||= merge_request.target_branch_sha
     self.head_commit_sha  ||= merge_request.source_branch_sha
     self.base_commit_sha  ||= find_base_sha