[FIX] purchase: update reference onchange of vendor_bill_purchase_id

[len-foss]

Commit 1be873d 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 (vendor_bill_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

Description of the issue/feature this PR addresses:

Current behavior before PR:

Desired behavior after PR is merged:

--
I confirm I have signed the CLA and read the PR guidelines at www.odoo.com/submit-pr

[len-foss]

Obviously commit 1be873d can be seen as lacking foresight, as only one way to create vendor bills was really tested.
Since the autocomplete is a technical field used to create vendor bills, it makes sense to add the reference modification in the onchange.
However it leaves other potential flows open for the need of similar fixes (although this has been added in v12 so things might not be so bad).
AFAICT, unfortunately we need to assume that, or we revert that initial commit entirely.

[nim-odoo]

Hopefully it is the right fix now :)

[len-foss]

robodoo r+

[robodoo]

Merged, thanks!