Luke Combs brought a very special guest — Tracy Chapman — to the stage of the Grammy Awards on Sunday night (Feb. 4), where the two performed a duet version of “Fast Car.” Chapman began the performance, strumming an acoustic guitar and smiling broadly as she sang the first verse. Then, the cameras panned over to Combs, who took over vocal duties and grinned back at Chapman. Combs has made no secret of how much he idolizes Chapman — he’s said that “Fast Car” was his first favorite song — and he was very much the starstruck fan onstage, mouthing Chapman’s verses along as he watched her sing. “It…