data row diffing UPDATE + INSERT DML generation fix

in the case of table row data comparising for DIFFING, the primary keys to use are table B
but the table B and A columns are reversed compared to data_rows_overlay() as the comparison is B vs A not base vs overlay (AvB)
so the columns to match as base and overlay are reversed, comared to other calls to data_row_overlay_primary_key_index()