CLEVELAND — When Brandon Belt is in business, so are the San Francisco Giants. After playing in only three spring training games, how long it would take Belt to heat up once the season began was an open question heading into opening day. Look no further than Belt’s batting line over the first week for the answer. And in a 4-2 win Saturday night against the Guardians, the Giants were in business when they needed it most. Belt drove home the Giants’ first two runs in the fifth, with a two-out single up the middle, then scored the go-ahead run after leading off the eighth with his second base hit…