Substantially modernize the "arc land" workflow
Summary: Ref T13546. This has a lot of dangerously rough edges, but has managed to land at least one commit in each Git and Mercurial.
Test Plan:
- Landed one commit under ideal conditions in Git and Mercurial.
- See followups.
Maniphest Tasks: T13546
Differential Revision: https://secure.phabricator.com/D21315