Add support for landing to hosted Mercurial repos.

Summary: I've kept this as close as possible to the Git version for ease of review and later refactoring of them both together. At minimum, the functions to get the working dir should probably be cleaned up one day.

Test Plan: Landed a revision.

Reviewers: epriestley, Blessed Reviewers

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Differential Revision: https://secure.phabricator.com/D7534