Checkout PR on top of latest `master`/`main` branch

[saschagrunert]

Hey, this is a feature request which may be useful from a test-strategy perspective. We could add an option to allow applying the PR patch to the latest master branch instead of just checking it out. This would allow us to verify the change after it has been merged on top of the latest master before actually merging it.

Surely, it would be an optional feature which should be disabled per default.