Fewer git processes per card

Each card that works in a pool of clones runs git when it starts, when its agent hands over, and when it lands. On Windows every git process can cost from fifty milliseconds to over two seconds. Obeya now asks git fewer times at each of these steps. A start skips the fetch when the clone already has everything the checkout has. The checks at a handover take a third of the calls they took before. The card starts and lands exactly as before.