I don’t know how much producer Robert Redford influenced “The Mustang” (2019) but it has a quiet poetry to it that I’ve felt in some of his other late-career works. The analogy at the root of the story is blatantly obvious. Convicts training mustangs. Wild horses, wild men. Both must be tamed, for their own good. Get it? Yet what could have been a cookie-cutter film is filled with nuance, much of it ugly, as one might expect in the real world of a state prison. But it’s the nuance, including the mournfully symphonic soundtrack, that will draw you in.