Add support for building from git.

This verifies pretty much the same rules as SVN in terms of having a clean working directory and merged with the origin.  For OSG repos, it additionally checks that git / SVN are not out of sync and that the branch rules are in effect.

Additionally, this adds a CLI option (--repo) which allows you to switch between osg / upcoming / hcc / uscms.

I hand tested the various synchronization rules and got as far as I could in testing with koji (ran afoul of UW's allowed_scms list).  Looking at the koji source code, I think everything is ready to roll.