Avoid copy a Position to get a move's san notation

In move_to_san() we create by copy a new position just
to detect if move gives check. This could be very costly in
line_to_san() that calls move_to_san() for every move, so
create the position only once and pass a reference to move_to_san()

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>