Consolidated credit notes linked to their invoice

Commit message

[FIX] account: link credit note to consolidated source invoice

When a credit note (out_refund) consolidates sale lines coming from
several orders, _set_reversed_entry fails to set reversed_entry_id. The
check 'credit_note.invoice_line_ids.sale_line_ids in
inv.invoice_line_ids.sale_line_ids' relies on recordset.__contains__,
which requires the left operand to be a singleton and otherwise returns
False. As a result reversed_entry_id is never set when the credit note
carries lines from more than one order, even though it reverses a single
consolidated invoice.

Localizations such as TicketBAI rely on reversed_entry_id to build the
refunded document reference and reject the refund otherwise with
"Cannot post a reversal document while the source document has not been
posted".

Use the <= subset operator instead, which behaves correctly for singleton
and multi-record sets alike.

MT-15939