Fast Company: Watch out, Volkswagen–your ironclad grip on the diesel car market may be about to weaken. Autobloggreen reports that a GM-brand diesel passenger car is coming to the U.S. sometime in the near future. GM Vice Chairman Tom Stephens delivered the information to the attendee’s of this week’s Directions in Engine-Efficiency and Emissions Research (DEER) conference in Detroit. According to Stephens, future GM diesels might be based on existing diesel powertrains manufactured by GM in other …