Summary:
Previously we had separate backup per working copy. That's very confusing
since all these working copies shares the same repo. This diff fixes it and
also adds config option to clean unnecessary working copy server-side.
Test Plan: Run infinitepush unittest
Reviewers: #mercurial, quark
Reviewed By: quark
Subscribers: rmcelroy, quark, mjpieters, #sourcecontrol
Differential Revision: https://phabricator.intern.facebook.com/D4876230
Signature: t1:4876230:1492025747:3579e5046efc2ed309044fc3335c36ac4f7bdd04
