[FIX] purchase: vendor reference

- Create a product A
  MTO, Buy
  Add a supplier S1 and set a specific reference
- Create a product B
  MTO, Buy
  Add a supplier S2 and set a specific reference
- Create a SO with A & B, confirm
- 2 RFQ are created

The first RFQ is created with the correct reference, but not the second
one.

This is because `display_name` depends on context keys: it is computed
once for both products with `partner_id` set as `S1`, while it should be
recomputed for product B with `partner_id` set as `S2`.

In order to avoid this, we use `with_prefetch()` to avoid reusing the
prefetched value.

opw-2176638

closes odoo/odoo#44564

X-original-commit: d96c1910be8f242455de9b443fc9db82155b4dd1
Signed-off-by: Nicolas Martinelli (nim) <nim@odoo.com>