On the surface, “Megabus,” a new single from harmonious and experimental San Francisco indie-rock trio French Cassettes’ forthcoming full length, Benzene (June 7 on Tender Loving Empire), is a twee, jangly, indie-folk strummer. But rather than heart-on-the-sleeve earnestness, Scott Huerta deploys his wry wit across a tune full of arch, apathetic lyrics. “Who do you love? Who’s getting drunk on …