Paramount has enough confidence in Tom Cruise’s Mission: Impossible 5 that it moved its release date way up, from Christmas to July 31. But the production recently shut down for a week or so in London because the film’s ending was deemed unsatisfactory. Director Christopher McQuarrie was given the extra time to work out a new and improved finale with a writer friend whose identity remains a mystery and who will neither be paid nor credited. It’s unusual and costly for a big-budget film to halt in the middle of production, especially one with a release date about five months away.