[FIX] purchase: confirm of purchase orders with kit boms

When using phantom/kit boms,
`action_confirm` on the move can lead to the move unlink,
as new moves with the different parts of the product have
been created.

See `_action_explode` in `mrp/stock.py`

The method `action_confirm` on `stock.move` returns the list
of remaining moves.

`force_assign` should therefore be performed on this new
moves list, returned by `action_confirm`, rather than
be performed on the moves list sent to `action_confirm`

opw-660722