update.py: Add a for-maintainers --merge=PR option
    
    We need to rebase all merge requests before merging them, to properly
    handle Flathub data updates. Asking contributors to do this would
    be annoying, and Pagure can't handle the special way we rebase and
    update data at the same time, so add a 'update.py --merge=PR' to
    rebase and merge a pull request.