SampleMVar won't work; between getting the current value and changing
it, another thread could made a change, which would get lost.
TMVar works well; this update situation is handled by atomic transactions.
Note that, since this always pushes branch synced/master to the remote, it
assumes that master has already gotten all the commits that are on the
remote merged in. Otherwise, fast-forward prevention may prevent the push.
That's probably ok, because the next stage is to automatically detect
incoming pushes and merge.
