SAN FRANCISCO — A $15 billion settlement over Volkswagen’s emissions cheating scandal cleared a key hurdle Tuesday, with a federal judge giving preliminary approval to a deal that includes an option for owners to have the carmaker buy back their vehicles. Attorneys for Volkswagen owners sought approval from U.S. District Court Judge Charles Breyer, who is overseeing consumer lawsuits and government allegations that the company’s diesel engines cheated on U.S.