Fix a couple regressions due to diff style changes, where diffs were shown unstyled:

Fix a couple regressions due to diff style changes, where diffs were shown unstyled:
* move the addition of common/diffs.css from showDiffPage() to showDiff() which catches most 'legit' cases
* hack it in to a couple of uses where getDiff() is used and manually spit out
* hack around the hacked-up custom diff table in Oversight... that should be refactored in the base class