Beneath his winsome twang and happy-go-lucky grin, rock ‘n' roll pioneer Buddy Holly was just as capable of darker material. Behold the cautionary character study “Midnight Shift,” in which late hours take an increasing toll on a couple, or the lonesome pining at the heart of “Blue Days, Black Nights.” And Holly gets surprisingly gritty when paying homage on “Bo Diddley,” brooding against the titular idol's signature beat.