* Cavendish now five shy of Merckx record * Van Avermaet retains yellow jersey (Adds quotes, details) By Julien Pretot MONTAUBAN, France, July 7 (Reuters) - Mark Cavendish cemented his status as the Tour de France's greatest sprinter when he claimed his 29th stage victory in the race in Thursday's sixth leg. The 142nd triumph of his professional career put the Briton in outright second place on the list of all-time Tour stage winners ahead of Bernard Hinault (28). Cavendish is now five shy of Eddy Merckx's record. More...