Filter diffs when filtering files in tree on merge requests

Problem to solve

Further details

When a merge request covers multiple domains (e.g. frontend and backend), you'll often multiple doing code reviews from different point-of-views. The frontend developers will look at all the JavaScript, and the backend developers will look at all the Go.

The improved file filter is great at filtering the file types, so you can click through them, but it'd be also good to filter out the non-matching diffs as well.

Proposal

What does success look like, and how can we measure that?

The same filtering logic for hiding files in tree would apply to the diffs, so that they aren't shown.