Documentaries about pop stars including M.I.A and British singer songwriter Ed Sheeran, as well as more esoteric indie artists such as The Go Betweens and The Slits, have been drumming up interest in the rock doc genre in Berlin. Buyers and sellers have told Deadline that there's growing interest in the genre following the success of films such as Asif Kapadia's Amy and Brett Morgen's Nirvana feature Montage of Heck. Steve Loveridge's film, Matanga/Maya/M.I.A, about Sri…