Two years ago, Travis Morrison tipped us off to the wonder that is the Dirty Projectors, telling us they saved his relationship with rock ‘n’ roll after he saw them play at the Black Cat. After last night, we know why. Frontman Dave Longstreth has assembled an unbelievably talented band, including three woman whose ability to harmonize a) with him, and b) with each other in peculiar keys is truly a work of fate. Or…