On October 17, 1974, Willie Nelson walked into a TV studio on the University of Texas at Austin campus, strapped on his guitar, and created a new city. The musician, 41 and without a gray hair on his head, was there to tape the pilot episode of a show called Austin City Limits in front of an intimate crowd of young hippies and rednecks, the types of people who enjoyed his music the most. Nobody knew it then, but this fledgling show would become the longest-running music TV program of all time, and Willie and his fans would change country music—and Austin—forever.On Thursday Willie played Austin City Limits again, celebrating the fiftieth anniversary of that first performance—and his status as an American hero. This time…