Create "master-green" branch which always points to the last green commit

Idea

A branch which is automatically updated to point at the last commit to pass the tests on master.

Why

Avoid failures when rebasing/merging master into a merge request.

Currently you can work on a feature only, wait 30m for the builds to run, hit failures, rebase onto master, come back to it the next day and still have failing tests unrelated to your code. Instead if you rebased onto master-green you could fix all conflicts and be confident that the builds will pass.

/cc @DouweM @rymai @rspeicher