hack the code table to display diff highlights in PR compare
    
    Previously the highlighting of diffs in the files changed
    tab of the PR page used pygments. with pygments removed,
    and using highlight.js everywhere else, diffs for commits
    were no longer highlighted.
    
    Using highlight.js in this view is a bit tricky without some
    major work to changing how inline comments work #3488, so this
    PR is a stop-gap to get highlighting of diffs here working until
    we can fix this properly.
    
    Fixes #3531