Implement update of ref when merge request code is reloaded

app/models/merge_request_diff.rb

@@ -160,9 +160,17 @@ class MergeRequestDiff < ActiveRecord::Base
   private
 
   def compare_result
+    @compare_result ||=
+      begin
         source_sha = merge_request.source_project.commit(source_branch).sha
 
-    @compare_result ||= CompareService.new.execute(
+        merge_request.target_project.repository.fetch_ref(
+          merge_request.source_project.repository.path_to_repo,
+          "refs/heads/#{merge_request.source_branch}",
+          "refs/merge-requests/#{merge_request.id}/head"
+        )
+
+        CompareService.new.execute(
           merge_request.author,
           merge_request.target_project,
           source_sha,
@@ -170,4 +178,5 @@ class MergeRequestDiff < ActiveRecord::Base
           merge_request.target_branch,
         )
       end
+  end
 end