The first time John Mellencamp ever talked to Willie Nelson was in mid-August of 1985, when Willie called Mellencamp’s Bloomington, Indiana, home after finishing a round of golf at a nearby course. One of the guys Willie’d just played with had turned him on to Mellencamp’s then-two-week old album, Scarecrow, and its angry, anthemic title track protesting the ongoing American farm crisis, “Rain on the Scarecrow.” Willie was thinking about putting on a show to raise money for farmers and was looking for a cohost. Mellencamp was the first artist he called.Despite the new record and his small-town Midwestern roots, Mellencamp was not a likely candidate. All-star benefits were becoming something of a trend, and Mellencamp, the iconoclast, had passed on all such invites. What’s…