The Met: Live in HD series will kick off its 2015-16 season at Guild Hall on Saturday at 1 p.m. with “Il Trovatore,” Verdi’s four-act tragedy. Its notoriously complicated plot includes revenge, mis- taken identity, rivalry between suitors, suicide, and execution, all pulled togeth- er by its “absolutely glorious” music, ac- cording to WQXR-FM radio’s “Opera in Brief.”