SAN FRANCISCO — Only one player compared to Brandon Belt in Tony Vitello ‘s eyes. Clayton Kershaw . “The easiest way to put it is he and Kershaw were arguably the best two lefties in the state of Texas and, therefore, at the time, the country, too, really,” the Giants manager recalled from when he recruited the Giants’ beloved first baseman to the University of Missouri. That’s right: First baseman. And in the conversation with Kershaw. Belt, 38, put a ribbon on a 13-year big-league career —