Before this change if -u/--update was in effect we compared the size
of the files to see if the transfer should go ahead. This was
comparing -1 with an actual size so the transfer always proceeded.
After this change we use the existing `sizeDiffers` function which
does the correct comparison with -1 for files of unknown length.
See: https://forum.rclone.org/t/sync-with-google-photos-to-local-drive-will-result-in-recoping/11605
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