fs/operations: Clear accounting before low level retry

Statistics of ransfers which were interrupted are not cleared before
retry iteration. These transfers completed with over 100 percentage.

This change clears transfer accounting before next retry iteration is
done in order to keep numbers in track.

Fixes #3861

fs/operations/operations.go

@@ -402,6 +402,7 @@ func Copy(ctx context.Context, f fs.Fs, dst fs.Object, remote string, src fs.Obj
 		// Retry if err returned a retry error
 		if fserrors.IsRetryError(err) || fserrors.ShouldRetry(err) {
 			fs.Debugf(src, "Received error: %v - low level retry %d/%d", err, tries, maxTries)
+			tr.Reset() // skip incomplete accounting - will be overwritten by retry
 			continue
 		}
 		// otherwise finish