marge-bot: use upstream gitlab repo
Currently, marge-bot source code is hosted in the freedesktop repo at https://gitlab.freedesktop.org/freedesktop/marge-bot. Webhooks trigger the various marge-bot releases to update and restart themselves whenever there is a push to the freedesktop marge-bot repo. The advantage of using the freedesktop repo, rather than the upstream gitlab.com repo, is that marge-bot updates can be reviewed and processed in batches so that the changes can be controlled and rolled back easily if a problem arises. The disadvantage is that the freedesktop repo is not often updated and has fallen about a year out of sync with upstream marge-bot.
This change simplifies the process of updating marge-bot by pulling directly from upstream while fixing the repository at a specific commit hash. Future marge-bot updates would be achieved just by changing the marge repo commit hash.
Related to marge-bot#2 (closed) and marge-bot!1 (closed)