Make it clearer that diffs can be collapsed
To make it obvious that diffs can be expanded or collapsed, we include a caret that points either right or down. This is in line with the new File Tree from https://gitlab.com/gitlab-org/gitlab-ce/issues/20962#note_14050626
The whole row can be clicked on, which toggles the expanded/collapsed states. This allows us to go back to embedding the file name with a link to that particular diff. When you hover over a row it drops a few shades to indicate you can click it.
Finally, I added color to the Click to expand line so it looks more like a link.
[Meta issue #20327 (closed)]