Unsaturated Fats Cut Gallstone Risk By BILL BERGSTROM    PHILADELPHIA (AP) -- A Mediterranean diet rich in vegetable oil, nuts and fish may help fend off more than heart disease and diabetes: It appears to prevent gallstones, too.    Men who ate more of the unsaturated fats found in such foods lowered their risk of gallstones by nearly one-fifth, according to a 14-year study published in Tuesday's Annals of Internal Medicine...