Diego Luna got his feet wet as a director with the promising 2011 debut,  Abel , a slight but disarming tragicomedy that took imaginative shots at Mexican patriarchy and manhood. He followed in 2014 with the larger-scale  Cesar Chavez , a pedestrian bio-drama that forfeited any shelf life it might have had when evidence accusing the iconic labor unionist of sexual abuse, grooming and rape surfaced earlier this year, all but obliterating his legacy. Next came 2016’s  Mr. Pig , a hog farmer road-trip movie with Danny Glover and Maya Rudolph which I flatly refuse to believe exists. Sadly, Luna’s inert fourth feature behind the camera,  Ashes  ( Ceniza en la Boca ), is unlikely to course-correct that faltering trajectory. Based on a well-regarded novel by Brenda Navarro, it’s a wafty character study so stripped down and elliptical that it lacks the connective tissue to hook us into its story or provide emotional access to its characters.…