Highlight auto-collapsed diffs in merge request diffs

Description

In merge request diffs large files are collapsed by default.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3270/diffs

That's great but while reviewing and scrolling through the changes I sometimes overlook a collapsed file.

Proposal

• Highlight auto-collapsed files so they can easily identified while scrolling through the changes.
• If a file is manually collapsed, hide everything except for the file title/header.

CleanShot_2020-09-04_at_11.46.15

old screenshot

Links / references

https://gitlab.com/gitlab-org/gitlab-ce/issues/14103

Release notes

Merge requests which apply changes to multiple files will sometimes collapse large file diffs in order to improve performance. When this happens, it's easy to overlook a file and not review it, especially in merge requests with many files.

GitLab will highlight diffs containing collapsed files so they are not missed during a review.