Since the self-cleaning model (Jan 10), polecats push branches to origin
before `gt done`. The refinery was only deleting local branches after
merge, causing stale `polecat/*` branches to accumulate on the remote.
Now deletes both local and remote branches after successful merge.
Uses existing `git.DeleteRemoteBranch()` function. Remote deletion is
non-fatal if the branch doesn't exist.
Fixes#359
2a0a8c760b