Support disk backed storage for repositories

[GeorgeMac]

Currently, we clone source projects into memory.

This is ok, but wont scale well for larger projects. We should support cloning to temporary or specified directories. Some care will likely need to be taken for concurrent working directories al the git index. Ideally, we can create a unique working index per request, but I need to do some more discovery into that (w.r.t go-git).