MR version comparison should filter changes from rebases

In my experience working on the code.videolan.org instance, it is quite common as an MR author to push changes that consist not just of modifications to the submitted work itself but also sometimes rebases upon changes that have been merged since the MR was submitted.

I have noticed that when you use the 'compare with previous version' link to view what changes have been made since the last push (as both MR author and reviewer), unhelpfully the diff includes all changes that come from rebases, making differentiating any changes the MR author has deliberately made from those changes that just from the rebase commits very difficult.

This is a very significant problem for being able to conduct reviews. MR authors mindful of this sometimes will make two pushes, one for the rebase and one for any other changes made to try to help reviewers, but this is obviously imperfect since it's impossible for the reviewer to know whether or not the rebase-only push really does just contain rebase-only changes that are safe to ignore; sometimes we forget to do it; and not everyone is mindful enough to do it.

The 'compare with previous version' feature desperately needs to be made more intelligent, filtering all changes that have purely come from the rebase only, thus revealing only the changes that a reviewer actually needs to consider (changes from resolution conflict and other changes pushed in combination with a rebase).

Obviously there's a complication with hiding rebase changes when a rebase required conflict resolution. The feature is going to have to understand when a difference involves lines changed by a rebase and present this in a suitable way such that reviewers can distinguish between the change from the previous version, the relevant change introduced by rebase commits, and the adjusted change in the new revision. Thus enabling reviewers to assess exactly what changed and why without the huge amount of additional noise that could potentially hide malicious/controversial changes that could slip past final review.