Fix funky diff padding to make diffs fully readable.

Test case: http://meta.brickimedia.org/index.php?title=Forum:Games_(2)_and_Answers_Wikis&diff=next&oldid=195077 (or basically any diff)

Tested in Internet Explorer 8.0 and Opera 20.0 (under WinXP) and this is definitely an improvement on both platforms.