Preheat a grill. Strip the rosemary branches of enough of their leaves to make room for the tomatoes, bread, and cheese. Chop the leaves and reserve. Alternating between each, thread the tomatoes, bread cubes, and mozzarella cubes onto the rosemary skewers (cutting the tip of the branch into a sharp point makes this easier). Drizzle with the olive oil and season with salt and pepper. Place the skewers on a cooler part of the grill and cook for 7 to 10 minutes, until the bread toasts and the cheese begins to melt. (This can happen very quickly if the grill is too hot. Either way, it's important to turn the skewers throughout so that all sides are exposed to the heat.) Serve with another drizzle of olive oil and garnish with the reserved rosemary leaves.