Faster status checks for huge git repos

The current status check for git repos can take several seconds when dealing with huge repos (WebKit in my case). It means a ~10 seconds delay for each command ran on the CLI.
Thanks to some twitter ranting, @paulirish and https://gist.github.com/3898739, I replaced `git status` with `git diff`.
AFAICT, `diff` based dirty/clean reports are identical to those of `status`, while being an order of magnitude faster (around 0.5 sec for the WebKit repo on my machine).