Pygments by default generate a series of wrappers around the highlighted content.
These wrappers were <div> and <pre> elements, as the output of Pygments is intended to go directly into another HTML element, and not manipulated.
Since we put them in our Diff Viewer, we should strip out those unwanted wrappers, as they break the Diff Viewer in some cases.

Tested with reviews available at reviews.reviewboard.org, in search for regressions, and found none.