More than 150 years since the end of the Civil War, slavery remains the scar that just won’t heal in the United States. Recognizing that, Antoine Fuqua’s “Emancipation” is a bracing and still-necessary attempt to face this painful legacy head-on, inspired by perhaps the single most powerful image we have of a once-enslaved person: the 1863 portrait of “Whipped Peter,” whose lacerated back served as shocking proof of unconscionable mistreatment by his white “masters.” That photograph, widely circulated during the Civil War, forced the world to confront the cruelty of this system — much as Fuqua himself does with this...