Add a pre-review hook to enforce better code review practices

With this hook you will not be allowed to submit new code whilst
there are still patches waiting for code review. Ironically I now
cannot submit this patch until I do some code review haha

(Note this hook can be overridden in exchange for bad karma - the
hook tells you how).

Change-Id: I5fee3afccb8481807a1362528765901f54988df7