The three New Hampshire sisters who formed a rock band because their father forced them to (when he died, in 1975, they stopped) developed a serious cult following, either despite or because of “deficits in what is commonly understood as standard musicality.” Five years ago, a Brooklyn musician did a Shaggs tribute concert; he’s since made meticulous transcriptions of their songs, and this summer he got the two surviving sisters to perform with him and his band at Mass MOCA. Howard Fishman was there.