[FIX] mrp: update unit_factor when updating MO quantity

Set a BOM to create 10^k units of Product P, using n units of raw material R.
On R, put rounding to 1 unit.
Create a MO to produce 1 unit of P.
Since it needs n/10^k units and rounding is set to 1,
it sets 1 unit of R to consume.
Now "update" (quantity to produce) to 10^k'.
It correctly updates the quantity to consume.
However, since it doesn't update the unit_factor, when you "produce",
it will consume 10^k' units instead of 1.

We copy the way the unit_factor is initially computed when updating the lines.

opw 1903776

closes odoo/odoo#29067

addons/mrp/models/mrp_production.py

@@ -440,6 +440,7 @@ class MrpProduction(models.Model):
         if move:
             if quantity > 0:
                 move[0].write({'product_uom_qty': quantity})
+                move.unit_factor = quantity / move.raw_material_production_id.product_qty
             elif quantity < 0:  # Do not remove 0 lines
                 if move[0].quantity_done > 0:
                     raise UserError(_('Lines need to be deleted, but can not as you still have some quantities to consume in them. '))