(Recasts after European markets open, changes dateline from previous SYDNEY)* Santa rally extends in Europe in thinning trade* Greece, China worries in background* Asian markets thin and sluggish as Tokyo on holidayBy Patrick GrahamLONDON, Dec 23 (Reuters) - The year-end rally on Europe&aposs major stock exchanges edged into its sixth day on Tuesday, just managing to ride out growing concerns over