Summary: Ref T13546. Fixes T2928. Adds a new "arc work" workflow which functions like the older "arc feature" workflow, but with modern infrastructure.
Test Plan: Used "arc work" to begin work on branches, bookmarks, and revisions in Git and Mercurial.
Maniphest Tasks: T13546, T2928
Differential Revision: https://secure.phabricator.com/D21336
