Re-insert my changes to the Diff engine that make it so that the original text is...

Re-insert my changes to the Diff engine that make it so that the original text is diffed
and only HTML-escaped on output. JeLuF was seeing unescaped HTML because he had
$wgUseExternalDiffEngine turned on; hence, this commit makes it so that pre-escaped text is
sent to such an external engine just as before. Also fixes indentation and readability
issues, adds a few comments, and removes trailing whitespace in only 4(!) lines. :)