message: fix: fix missing blobs for upgrade-backstage-app
  3 way merge was failing because of missing ancestors.
  Fetching the upgrade-helper-diff repository as one of your app's remotes fixes the problem.
  The upgrade-helper-diff contains a branch for each version with a clean app bootstrapped with the create-app script.
diff: @@ -9,6 +9,8 @@ if [ "$CURRENT_VERSION" == "$TARGET_VERSION" ]; then
  else
  echo "Attempting upgrade from Backstage $CURRENT_VERSION (create-app $CREATE_APP_CURRENT_VERSION) to $TARGET_VERSION ($CREATE_APP_TARGET_VERSION)"
  rm -rf .upgrade && mkdir .upgrade
  + git remote add upgrade-helper https://github.com/backstage/upgrade-helper-diff.git
  + git fetch upgrade-helper
  curl -s https://raw.githubusercontent.com/backstage/upgrade-helper-diff/master/diffs/$CREATE_APP_CURRENT_VERSION..$CREATE_APP_TARGET_VERSION.diff > .upgrade/upgrade.diff
  git apply -3 .upgrade/upgrade.diff
  git mergetool
  