When current post-review script encounter git repository, it always runs git svn info. This is undesirable for people who have git-svn installed(and perhaps active) but want to generate diffs against a git repository. In these condition git svn info also seems to hang for a very long time, which makes post-review unusable in these installations.

This patch add --git-only option that skips git-svn calls and proceeds directly to git-specific stuff when initializing GitClient.

Seems to work: no "git svn info" happens, diffs flow smoothly into git-bound RB installation.