“One day I’ll be a fallen ‘It Girl’,” says Greta Gerwig, discussing her recent rise from the American movie underground to the rapture of mainstream Hollywood critics. A key figure in cinema’s so-called ‘mumblecore’ movement, Gerwig starred in such no-budget films as LOL, the Duplass brothers’ Baghead, and Hannah Takes the Stairs (which she co-wrote), while co-writing and directing her own feature, Nights and Weekends. Following a part in last year’s retro horror The House of the Devil, Gerwig broke on to the indie major scene with a lead role as young Los Angeles transient Florence, opposite Ben Stiller, in…