Foo Fighters returned to the stage last night in San Francisco as a part of the Acoustic-4-A-Cure benefit show and decided to debut a new song from their yet-to-be-announced new album. “The Sky is a Neighborhood” harkens back to a sound heard on 1997’s The Colour and the Shape, with a mix of blues and rock that the group since traded in for a more straightahead, garage-rock sound fit for…