Do not create a pipeline on MR refresh if source branch was deleted

Merge branch 'security-mr-delete-source-branch-no-pipeline-17-4' into '17-4-stable-ee'

See merge request gitlab-org/security/gitlab!4522

Changelog: security

app/services/merge_requests/refresh_service.rb

@@ -44,7 +44,7 @@ def refresh_merge_requests!
         mark_mr_as_draft_from_commits(mr)
         execute_mr_web_hooks(mr)
         # Run at the end of the loop to avoid any potential contention on the MR object
-        refresh_pipelines_on_merge_requests(mr)
+        refresh_pipelines_on_merge_requests(mr) unless @push.branch_removed?
         merge_request_activity_counter.track_mr_including_ci_config(user: mr.author, merge_request: mr)
       end

spec/services/merge_requests/refresh_service_spec.rb

@@ -246,6 +246,18 @@
           expect(@another_merge_request.has_commits?).to be_falsy
         end
+        context 'when push is a branch removal' do
+          before do
+            # If @newrev is a blank SHA, it means the ref has been removed
+            @newrev = Gitlab::Git::SHA1_BLANK_SHA
+          end
+
+          it 'does not create detached merge request pipeline' do
+            expect { subject }
+              .not_to change { @merge_request.pipelines_for_merge_request.count }
+          end
+        end
+
         context 'when "push_options: nil" is passed' do
           let(:service_instance) { service.new(project: project, current_user: @user, params: { push_options: nil }) }