+'GetDifferenceEngine': Called when getting a new difference engine interface object
+Return false for valid object in $differenceEngine or true for the default difference engine
+$context: IContextSource context to be used for diff
+$old: Revision ID to show and diff with
+$new: Either a revision ID or one of the strings 'cur', 'prev' or 'next'
+$refreshCache: If set, refreshes the diff cache
+$unhide: If set, allow viewing deleted revs
+&$differenceEngine: output parameter, difference engine object to be used for diff
+