core-pr-tester should have an option to merge into master first, and fail on conflict.

This would allow allow to test even older PRs, that are otherwise fine (i.e. no conflicts), on the latest master. I had to manually merge and update https://github.com/jenkinsci/jenkins/pull/2844 earlier because I wanted to run it on the latest version rather than its 2 years old HEAD. It would be much easier if the core-pr-tester would do that for me.