Filefield upgrade enters infinite loop while updating nodes with more than 50 attachments

Just ran into a problem with update 6001 in the _filefield_update_6001_move_operation function. Using the Batch API, this update selects 50 nodes at a time to process with the following query (line 311):

$result = db_query_range("SELECT * FROM {{$context['sandbox']['table']}} WHERE vid >= %d ORDER BY vid ASC", $context['sandbox']['current_node'], 0, $limit);

In cases where the number of attachments is greater than 50, no progress is made between batches because current_node doesn't increase. This patch is a first pass at tracking the delta column along with "current_node" to allow the update to complete.