In "arc land" in Mercurial, inch closer to making complex branch/bookmark workflows function

Summary:
Ref T9948. Ref T13546. This change moves toward a functional "arc land" in Mercurial.

Because of how "bundlerepo.getremotechanges()" works, "hg arc-ls-markers" does not actually list markers in the remote that aren't different from local markers so it's hard to get anywhere with this.

Test Plan: Got somewhat-encouraging output from "arc land" and "hg arc-ls-markers", but too many things are still broken for this to really work yet.

Maniphest Tasks: T13546, T9948

Differential Revision: https://secure.phabricator.com/D21348