IN the late 1960s, Willie Nelson spent $150 on a new Martin acoustic guitar that he named Trigger, after Roy Rogers’ horse. He still plays it more than 50 years later. It’s a story that’s been told many times, but it takes on new significance in , an upcoming new five-part documentary by co-directors Oren Moverman (, ) and Thom Zimny (Bruce Springsteen’s go-to
