Allow GitLab’s rebase button to work in some cases

GitLab’s merge methods involving fast-forwarding require the branch to be rebased before merging via the UI. The target branch must be an ancestor of the merge request’s branch. This gets annoying because the target branch is a moving target. The maintainer will need to be able to operate the rebase button to get the history in line.

We have been using GitLab’s protected branches, because they generally make the UI more straightforward. However, they cause the rebase button to fail with an unclear error message. We have to unprotect all the branches in issue fork repositories.

Branches are still protected by our update hook. This can currently detect and allow straightforward rebases. https://bitbucket.org/drupalorg-infrastructure/drupal-git-hooks/src/7ed6...