Show message when MR diffs are outdated
On the MR page, we make an AJAX request to get the diffs. If you're on the conversation tab, and someone pushes changes, the diffs will be different to the ones you would have seen on page load. This is exacerbated by https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4990 which adds a potential AJAX request for each file in an MR.
We should add some params for the diff comparison to these requests, and then if those in the response don't match, we can show the use a message saying the diff is outdated.
This also applies to the compare view, but I'm not sure if we care so much about this there.