Use CI_COMMIT_REF_NAME to detect branch name

Since CI_COMMIT_BRANCH does not exist for merge request pipeline

.gitlab/ci/notify.gitlab-ci.yml

@@ -10,7 +10,7 @@ notify-update-gitaly:
   extends:
     - .notify-slack
   rules:
-    - if: '$CI_MERGE_REQUEST_IID && $CI_COMMIT_BRANCH == $GITALY_UPDATE_BRANCH'
+    - if: '$CI_MERGE_REQUEST_SOURCE_BRANCH_NAME == $GITALY_UPDATE_BRANCH'
       when: on_failure
       allow_failure: true
   variables: