Teach foliage to fetch git sources using git

[michaelpj]

At the moment, if you have a git repository R with N packages, and you make M releases of the packages, then we will need M*N tarballs of various states of R. This can take a while to download (we can't save ourselves from the space usage since I think we need the tarballs in the end anyway...). It would be more efficient to just clone R and then make tarballs locally.