merge tags function hangs with unique index conflict

when we merge tags, we cycle through a series of sql statements (CRM_Core_BAO_EntityTag::mergeTags())

the first statement updates all entities assigned to tag A to be assigned to tag B
however – we have a unique index constraint on entity_table, entity_id, tag_id. if a contact is originally tagged with BOTH tag A and B, that first UPDATE statement fails with the index constraint and the merge hangs.