3447: update review by commit

danger******@gmai***** (Google Code) (Is this you? Claim this profile.)
What version are you running?
RB 2.0.2

What's the URL of the page this enhancement relates to, if any?

Describe the enhancement and the motivation for it.
Since RB 2.0 you can choose to start a review on a existing commit. For that post-commit workflow it would be nice to also update reviews based on a following commit, so you don't need to create a diff first and then upload it.
Currently you can put a commit number in that dialog, but without a diff, it seems to stuck in a loop

What operating system are you using? What browser?
debian testing / chrome

Please provide any additional information below.