RFC: Add git support for jail command

I have a patch here that adds preliminary support for git as a source for FreeBSD source tree.

**This is not intended to be taken as-is, as this is only for comment/suggestions.**

If there is interest and feedback I can rebase and fix this against master branch.

I can also see what else needs to be done, as I've only just barely started work on this.

(One trick might be to set --depth for the git clone to make it faster, I will look into that.)