This song changed James Mercer’s life. Fed up with the Albuquerque music scene, which he found “macho, really heavy, and aggressive,” as he put it in a 2012 interview, Mercer picked up an acoustic guitar and wrote “New Slang,” a relentlessly catchy indie-folk tune in the bright key of C major. At that point in his life, his future band, the Shins, didn’t have an album yet. Since 1992, he’d been singing and playing guitar in Flake Music, a group …