A federal judge granted class-action status to Wal-Mart Stores Inc. shareholders who filed a lawsuit in 2012 alleging the retailer defrauded investors by covering up a bribery scheme in Mexico. In a Fayetteville ruling earlier this week, U.S. District Court Judge Susan Hickey rejected Wal-Mart’s argument that the City of Pontiac General Employees’ Retirement System — a Michigan-based …