[FIX] purchase: move reference update to the right onchange

Move the code from commit ea1fc124 to the onchange on purchase_id.
The field vendor_bill_purchase_id is a technical one added in v12 to allow using
old vendor bills as template for new vendor bills.
However the logic on purchase_id was already in v11, so we match commit 14239733

opw 1962364

closes odoo/odoo#32661

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

addons/purchase/models/account_invoice.py

@@ -25,10 +25,6 @@ class AccountInvoice(models.Model):
             return {}
         self.purchase_id = self.vendor_bill_purchase_id.purchase_order_id
         self.vendor_bill_id = self.vendor_bill_purchase_id.vendor_bill_id
-        vendor_ref = self.vendor_bill_purchase_id.purchase_order_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
         self.vendor_bill_purchase_id = False
         return {}
 
@@ -98,6 +94,11 @@ 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
+
         if not self.invoice_line_ids:
             #as there's no invoice line yet, we keep the currency of the PO
             self.currency_id = self.purchase_id.currency_id