Manual:Hooks/GetDifferenceEngine

Details
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