[FIX] sale: when creating a refun from an invoice, the invoice lines must be linked to the original so lines to compute the invoiced quantity correctly

Since invoice quantities are computed from the invoice lines linked to the so line via a many2many field, we need to overload the refund function that adds the fields on the invoice lines; explicitly copying the sale order lines references.

addons/sale/sale.py

@@ -785,6 +785,13 @@ class AccountInvoice(models.Model):
             order.message_post(body=_("Invoice %s paid") % (name))
         return res
 
+    @api.model
+    def _refund_cleanup_lines(self, lines):
+        result = super(AccountInvoice, self)._refund_cleanup_lines(lines)
+        for index, line in enumerate(lines):
+            result[index][2]['sale_line_ids'] = [(6, 0, line.sale_line_ids.ids)]
+        return result
+
 
 class AccountInvoiceLine(models.Model):
     _inherit = 'account.invoice.line'