Do not create a MergeRequestDiff record when source branch is deleted

In order to have a valid MergeRequestDiff record when need head_commit_sha.
When a source branch is deleted head_commit_sha is nil.
This causes an exception in merge request "Changes" tab.

1 files changed, 10 insertions, 0 deletions

diff --git a/spec/services/merge_requests/refresh_service_spec.rb b/spec/services/merge_requests/refresh_service_spec.rb
index e515bc9f89c..0220f7e1db2 100644
--- a/spec/services/merge_requests/refresh_service_spec.rb
+++ b/spec/services/merge_requests/refresh_service_spec.rb
@@ -227,6 +227,16 @@ describe MergeRequests::RefreshService, services: true do
       end
     end
 
+    context 'when the source branch is deleted' do
+      it 'does not create a MergeRequestDiff record' do
+        refresh_service = service.new(@project, @user)
+
+        expect do
+          refresh_service.execute(@oldrev, Gitlab::Git::BLANK_SHA, 'refs/heads/master')
+        end.not_to change { MergeRequestDiff.count }
+      end
+    end
+
     def reload_mrs
       @merge_request.reload
       @fork_merge_request.reload