Four stars. Rated PG-13. 125 minutes. Until this year, perhaps the greatest piece of moviemaking about Dunkirk was only part of a movie: It was a breathtaking sequence of the massive World War II evacuation, filmed in one astonishing five-minute take that dramatically punctuated the movie “Atonement,” directed by Joe Wright. Now Wright returns with a fully fledged Dunkirk film: “Darkest Hour” is already receiving awards chatter for Gary Oldman’s deliciously crafty portrayal of the film’s main subject, a newly minted British prime minister named Winston Churchill.