Always amend to the same commit for gh-pages (paritytech#5909)

.gitlab/pipeline/publish.yml

@@ -54,7 +54,7 @@ publish-rustdoc:
     # This causes GitLab to exit immediately and marks this job failed.
     # We don't want to mark the entire job failed if there's nothing to
     # publish though, hence the `|| true`.
-    - git commit -m "___Updated docs for ${CI_COMMIT_REF_NAME}___" ||
+    - git commit --amend -m "___Updated docs" ||
       echo "___Nothing to commit___"
     - git push origin gh-pages --force
     # artificial sleep to publish gh-pages