Provide a more powerful query mechanism for "markers" (branches/bookmarks)
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