Mergeability status not always up to date, CI may build older detached ref

[mmitche]

If a PR had merge conflicts or if GH was slow in updating mergeability status, the system may determine that a PR is not mergeable. In this case, it will build a detached head instead of the PR merge branch. This is the correct behavior. However, once mergeability is fixed, this should switch over to using the merge branch again.

This should be fixed by https://github.com/jenkinsci/ghprb-plugin/pull/365. However this is not rolled out

In the meantime, it's possible to workaround this issue by fetching and merging the upstream target branch yourself, then pushing that new ref.