(bug 34929) Show the correct diff when a section edit is rejected by the spam filter...

(bug 34929) Show the correct diff when a section edit is rejected by the spam filter. Use EditPage::showDiff() so that the same kind of diff is shown as when you click the "show changes" button. I considered changing the source text to be the original section text, but if you do that, the line numbers are wrong and section=new doesn't show you the headline which may have matched the spam filter.