Nitty Gritty Dirt Band scored its first charting single in 1967, “Buy for Me the Rain,” and embarked on a self-propelled ride through folk, country, rock ‘n’ roll, pop, bluegrass, and the amalgam now known as “Americana,” producing timeless classics like “Fishin’ in the Dark,” “Mr. Bojangles,” and “Will the Circle Be Unbroken.”