marge-bot-mesa: use --add-reviewers/keep-reviewers
Currently developers have to manually add "Reviewed-by" to their commit messages. Implement the --add-reviewers option for the mesa/mesa project so that marge will add a Reviewed-by line to the commit for everyone who clicked "Approve" for an MR. The --keep-reviewers option makes sure to keep any "Reviewed-by" lines that the developer added manually.
IRC discussion: https://people.freedesktop.org/~cbrill/dri-log/index.php?channel=freedesktop&date=2024-10-16