[Fix] edithistory compare revisions

You can see revisions but not compare them:

This works:
http://forum.xbmc.org/edithistory.php?pid=1557328
but when you click one of the 'compare to current post' buttons you get:
http://forum.xbmc.org/edithistory.php?action=compare&pid=1557328&eid=168044
(blank page)