One way to quickly create a PR is to open a release branch, and click "... commits behind master":

That will generate a PR that has every commit since diverged, like so:

#166874

Unfortunately it doesn't work as expected, because it thinks this is really the previous commit:

DetailedApiRequestError(status: 409, message: Document already exists: projects/flutter-dashboard/databases/cocoon/documents/ciStaging/flutter_flutter_2fc716dbeb68abf9675d7f81162b0b90ceb88808_engine)

This would be a huge time saving workflow to be able to use, so it's unfortunate it doesn't work.

/cc @jtmcdole @reidbaker