Consider refactoring trebuchet to use dulwich or pygit2

[ryan-lane]

We currently shell out to git rather than using the library support that salt uses elsewhere. Dulwich or pygit2 are useful for native library support, exceptions, and flexibility. This does, however, add a pretty stiff learning curve for anyone wanting to modify trebuchet, so that alone may make this a bad idea. Trigger is currently using GitPython with plans on moving to pygit2 or dulwich.