[FIX] purchase: update reference onchange of purchase_id

Commit 1be873d4 removed the modification of the reference field
from the _onchange_origin.
The reasoning was that since this is an onchange on invoice_line_ids, it was
applied too often, which annoyed users in the case they overwrote that field.

However when using the "Autocomplete" field (purchase_id),
then the user expects values to by modified.
Before, it worked by setting the origin field, which trickled down
to the reference.
Since that commit removed this, we re-add it explicitly in that case.

opw 1962364

closes odoo/odoo#32656

Signed-off-by: Nans Lefebvre (len) <len@odoo.com>

addons/purchase/models/account_invoice.py

@@ -79,6 +79,11 @@ class AccountInvoice(models.Model):
         if not self.partner_id:
             self.partner_id = self.purchase_id.partner_id.id
 
+        vendor_ref = self.purchase_id.partner_ref
+        if vendor_ref:
+            self.reference = ", ".join([self.reference, vendor_ref]) if (
+                    self.reference and vendor_ref not in self.reference) else vendor_ref
+
         new_lines = self.env['account.invoice.line']
         for line in self.purchase_id.order_line - self.invoice_line_ids.mapped('purchase_line_id'):
             data = self._prepare_invoice_line_from_po_line(line)