Gitfiletree

Hi Dale,

a more interesting pull request. The last version of MonticelloFileTree-Git is able to scan all package versions inside the repo via git, and load any of them.

It request a zip archive from git for a specific commit, and load the definitions from that zip archive. There are still untested stuff (I'm not too good on all operations you can do on a version or a repository) but most of it looks fine.