checkupdates: --merge-request flag for single app runs then a MR

This is a new feature to be used in the checkupdates Buildbot setup (currently at https://checkupdates.at.or.at/, hopefully soon to be https://checkupdates.f-droid.org). Buildbot will run fdroid checkupdates per ApplicationID. This new feature will push any new commits and then open a merge request. To avoid having thousands of branches on checkupdates-bot's fork, this cleans up any remote branches that are pointing to commit that has been fully merged.

FYI @cde