Xavier Beauvois’s drama is about a band of Cistercian monks in the Algerian mountains in the 1990s who must decide whether or not to flee their monastery after Islamist militants have invaded the area. A film of almost supernal grace, it closes with a sequence I will never forget, as the monks share a makeshift Last Supper while Tchaikovsky’s grand theme from “Swan Lake,” a favorite piece of music, plays on the Victrola.