Kevin, it should be totally possible to do what you're describing, assuming that at least one side of the VPN has the ability to forward ports in. If both VPN endpoints are behind one-to-many NATs then you're going to need something more than merely tinc.
If, however, you can forward ports in on one of your endpoints then you're in luck. Here's a write up that I put together explaining how to get tinc working on Ubuntu, Windows 7 and Centos. This covers all of the tinc setup but not how to establish port forwarding.
Once you have tinc setup and working on both ends, you'd simply make your git calls to the VPN network IP (something like 172.20.20.100 in my examples) :
git clone firstname.lastname@example.org:reponame.git