Citi strategists on Thursday urged European firms to use historically cheap borrowing costs to buy back their own shares, a practice embraced by U.S. companies in recent years that has also attracted criticism. What European companies should do with their trillion-dollar cash pile in a world of rock-bottom interest rates and cheap central-bank money has become a hot topic for investors, as carrying cash on balance sheets becomes costlier while borrowing funds for everything from mergers to buybacks becomes cheaper.