The libgit2-based git2go is a decent API, but it's a pain to deal with.

It needs very specific versions of libgit2 to be installed, and makes it

very hard to cross-compile or distribute binaries that link against it.

This change ports rb-gateway to the go-git library, which is a pure Go

implementation with a very similar API to git2go.

Ran unit tests
Tested post-commit review request creation and diff viewing with

  Review Board.