[FIX] sale: customer reference index

- Create hundreds of SO with a Customer Reference set
- Select them all
- Click on Action Create Invoices

The following error is raised:

```
psycopg2.OperationalError: index row size 5480 exceeds maximum 2712 for index "account_move_line_partner_id_ref_idx"
```

The issue is similar to c545783b95, so is
the solution.

opw-2180139

closes odoo/odoo#43906

X-original-commit: dbd1858b623c5bf4c14c8de568c6addc7868b14e
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>

addons/sale/models/sale.py

@@ -628,7 +628,7 @@ class SaleOrder(models.Model):
                     payment_refs.add(invoice_vals['invoice_payment_ref'])
                     refs.add(invoice_vals['ref'])
                 ref_invoice_vals.update({
-                    'ref': ', '.join(refs),
+                    'ref': ', '.join(refs)[:2000],
                     'invoice_origin': ', '.join(origins),
                     'invoice_payment_ref': len(payment_refs) == 1 and payment_refs.pop() or False,
                 })