Summary:
This adds the ability to specify a config file to be used during the
command. This is useful during clones for letting the clone command use the
given repositories system specified repo-specific hgrc file.
Reviewed By: quark-zju
Differential Revision: D7311576
fbshipit-source-id: a97d8ebada2e0bea27c75a7650df8ede00dc10c6
