Improve diff readability with inline diff highlighting

[JanTvrdik]

The difference readability should be increased with inline diffs using probably the same style as https://github.com/git/git/tree/master/contrib/diff-highlight

[milo]

I would stay with yellow. IMHO, when showing diff, there is no reason for red/green. Cannot say old/new, correct/wrong because maybe test needs to be updated.

The inverse of difference may look nice 👍