Add basic support for mirroring Mercurial repositories

Summary:
Ref T4338. Mercurial exits with exit code 1 and "no changes found" in stdout
when there's no changes. I've split up the `pushRepositoryToMirror` to make
the code a tad more readable.

It isn't perfect, but it works for me.

Test Plan:
pushed some changes to my hosted repo. Saw them appearing in the
mirrored repo

Reviewers: epriestley, #blessed_reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4338

Differential Revision: https://secure.phabricator.com/D8107