Make Mercurial use "hg shelve" and "hg unshelve" in dirty working copies in "arc land"
Summary: Ref T13546. Implement the equivalents of "git stash" in Mercurial.
Test Plan: Dirtied a working copy in Mercurial, ran "arc land", saw dirty changes survive the process.
Maniphest Tasks: T13546
Differential Revision: https://secure.phabricator.com/D21329