[ab61] Direct debit orders do not always include the invoice number as reference

[Stefan Rijnhart (Opener)]

By default, payment orders take the move reference as the reference for the payment order line. That is fine to process supplier invoices but direct debit orders, which are built on the payment order process customer invoices. Customer invoices generate moves that have the invoice description as its reference if it is filled, and the invoice number only as a fallback if the reference is not filled. As a consequence, direct debit orders can have a reference that is not the invoice number.