Currently _action_assign will try to add new reservation to existing
move lines. With mrp it could be problematic with the following usecase:
- 2 product A in stock
- Create a MO for 2 final product tracked and consuming 4 products A
- Reserve -> It will create a ml with 2 product A.
- Product -> It will fill the ml with 2 quantity done for first lot
- Add 2 product A in stock (4 in total)
- Reserve -> It will increase the reservation on the existing move line.

It's a problem since the new produce (wizard or wo) will create a new
move line because the existing move line have a lot_produce_id and can't
be used for a different lot/SN anymore.

This commit only increase reservation if the move line has not been used in
order to create a lot.