Added automatic stash / unstash support for Git in arc diff.

Summary:
- Added arc.autostash option to have this behaviour off by
default (but configurable on a per-project basis).
- Automatic stashing of changes now informs the user of how
to restore their working directory if Arcanist unexpectedly
terminates.
- Fixed an issue with finalizeWorkingCopy when the workflow
didn't require a clean working copy.

Test Plan:
Test `arc diff` when there are changes in the working
directory; by default it should tell you to stash or commit.
Turn on the arc.autostash option and try again; it should
automatically stash with a message on how to recover, and
it should restore the working directory automatically under
almost any circumstances (other than an unrecoverable error).

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D5385