Before this change --compare-dest and --copy-dest would check to see
if the compare/copy object existed first, before seeing if the
destination object was present.
This is inefficient, because in most --copy-dest syncs the destination
will be present and the compare/copy object need never be tested.
--compare-dest syncs may also be speeded up if they are done to the
same directory repeatedly.
This fixes the problem by re-arranging the logic so if the transfer is
not needed then the compare/copy object is never tested.
See: https://forum.rclone.org/t/union-with-copy-dest-enabled-is-slower-than-expected/32172
a875320e37