--git-only option to make post-review ignore git-svn

Review Request #1378 — Created Feb. 1, 2010 and discarded

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.
  1. I don't like this approach. I'd rather be better about detecting git-svn repositories instead of requiring users to specify.
    1. In this particular situation the problem is more about detecting when not to use git-svn, but I agree that there are possible ways to detect that (for example checking whether upstream of given branch is in git-svn). For me this was more like a fast way to prevent getting bored by infinite(?) wait for git svn info to complete. Anyway, thank you for reviewing my patch, hopefully I'll write a better version some time.