Allow repository mirrors even if the repository is hosted elsewhere.

Summary:
I'm not sure if this is desired functionality, but we happen to need
mirroring of our repository which is not hosted by phabricator, and as far as
I can tell the mirroring code does not depend on the hosting code.

Test Plan:
Setup mirror with SSH credentials to github, pushed changes to
elsewhere hosted repository, commits got mirrored to github.

Reviewers: epriestley

Reviewed By: epriestley

CC: Korvin, epriestley, aran

Maniphest Tasks: T4338

Differential Revision: https://secure.phabricator.com/D7637