remote: honour Deepen/Unshallow precedence in Fetch

Trail finding (low): when opts.Unshallow was true but the repo was not shallow, the --unshallow switch case fell through to the Deepen case, so --deepen fired even though Deepen is documented as ignored whenever Unshallow is set. (Harmless in practice — git treats --deepen on a non-shallow repo as a no-op — but it contradicts the contract.) Guard the Deepen case with !opts.Unshallow so the precedence holds regardless of shallow state.