DETROIT — General Motors Co. and a battalion of trial lawyers are preparing for an epic court fight over whether GM is liable for the sins of its corporate past. The company is asking a U.S. Bankruptcy Court to shield it from legal claims for actions that took place before the company’s 2009 bankruptcy. But lawyers who are suing GM say it shouldn’t get the usual benefits of bankruptcy protection because it concealed a deadly ignition switch problem when the court was making bankruptcy decisions …