Fix 1px white line below file header in diff view

The following discussion from gitlab-ce!30927 should be addressed:

• @mikegreiling started a discussion: (+5 comments)

Due to the way gitlab-ce!30927 had to be implemented, a single 1px table row had to be added to the file view in order to preserve column widths when there is a “diff expansion row” at the beginning of a diff section.

We ought to explore whether there are better ways around this. Perhaps we could also conditionally hide this row when there is no diff expansion row