Summary: Adds the same check code test that upstream Mercurial uses.
Test Plan:
Ran it, and fixed all the failures. I won't land this commit until
all the failure fixes are landed.
Reviewers: #sourcecontrol, ttung, rmcelroy, wez
Reviewed By: wez
Subscribers: quark, rmcelroy, wez
Differential Revision: https://phabricator.intern.facebook.com/D3221380
Signature: t1:3221380:1461802769:19f5bdc209c05edb442faa70ae572ce31e2fbc95
