Review diff between pull request updates (BB-9427)

Pull requests are great, and so is the review process with its comments, notifications and the ability to update pull requests. However, when a pull request is updated, I would like to review just the changes introduced in the update. I think this could be done with an interdiff (show a diff between the previous and the current state of the pull request branch) or by color coding (for instance: the newest changes have a green/red background like the normal diff, older changes have a faded green/red background).