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