operations: fix -u/--update with google photos / files of unknown size

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

fs/operations/operations.go

@@ -1527,7 +1527,7 @@ func NeedTransfer(ctx context.Context, dst, src fs.Object) bool {
 		case dt <= -modifyWindow:
 			fs.Debugf(src, "Destination is older than source, transferring")
 		default:
-			if src.Size() == dst.Size() {
+			if !sizeDiffers(src, dst) {
 				fs.Debugf(src, "Destination mod time is within %v of source and sizes identical, skipping", modifyWindow)
 				return false
 			}