Add native git support for Mercurial repos.

git-remote-hg 2012.11.13 (new formula)
Adds support for Felipe Contreras' native git/Mercurial bridging
interoperability extension for git.  This will allow you to use
git against a Mercurial repository (with caveats noted on the
home page) without any major loss of functionality or compatibility.
We require a versioned tarball.