Summary:
Ref T13546. Various Arcanist workflows, and particularly the MercurialAPI, currently repeat quite a lot of code around parsing branches and bookmarks.
In modern Mercurial, we can generally use the "head()" and "bookmark()" revsets to do this fairly sensibly.
This change mostly adds //more// code (and introduces "arc bookmarks" and "arc branches" as replacements for "arc bookmark" and "arc branch") but followups should be able to mostly delete code.
Test Plan: Ran "arc branches" and "arc bookmarks" in Git and Mercurial.
Maniphest Tasks: T13546
Differential Revision: https://secure.phabricator.com/D21333
