Add two PO with references 'XY' and 'Z' to a vendor bill.
The reference is computed as 'XY, Z' (with ', ' as separator).
Add a PO with reference 'X', the reference becomes 'X' instead of 'XY, Z, X'.
There are two variables that may not be defined (vendor_ref and self.reference),
and specifically the case where a reference is the prefix of another one is not
correctly dealt with.
opw 2005825
closesodoo/odoo#33841
Signed-off-by: Nans Lefebvre (len) <len@odoo.com>