Feature: check backports have a "cherry picked from commit" line with valid hash

[Frederik Rietdijk]

1. Check whether all the commits in PR's targetting staging-* and release-* have a cherry picked from commit line
2. Check whether the mentioned hash is indeed in master.

There are cases where we do not backport, in which case ofborg will complain, but I think that is acceptable.

This is not very important, but still useful.