3e82395306
when we need to apply trailers to a whole MR at once, it's convenient to do something like `git rebase --exec "rb faith" origin/main`. This adds that operation into the script itself, so that can be done with simply `rb -r origin/main faith`. Signed-off-by: Alyssa Rosenzweig <alyssa@rosenzweig.io> Reviewed-by: Mel Henning <mhenning@darkrefraction.com> Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/34409>