This change adds the beginning of Git support to post-review. Right now, it only generates diffs for repositories against SVN remotes and only generates a single squashed change rather than a patchset (as Review Board doesn't yet support this). However, it's a step. Additional features will likely require changes in Review Board, which may wait until 2.0. This does make my life easier now, though :)

We scan to see if this is a repository against an SVN remote and then generate a diff in the format that `svn diff` would generate.

I posted this review request!