SAN DIEGO (Reuters) – Sylvester Stallone, Arnold Schwarzenegger and their fellow aging action heroes are back in fighting form for new movie "The Expendables 2," thrilling crowds at the giant Comic-Con pop culture event with footage from the upcoming film. The new movie, a sequel to 2010's surprise hit about a group of rebel mercenaries out to kill a brutal military leader, appeared similar to the first one in film clips shown Thursday with an emphasis on old-school shoutouts over sci-fi fantasy. …