Use also cost price for customer credit note

The unit price for incoming product from customer contains the sale price and
not the actual cost price.

issue4097
review12521002
(grafted from 704e00560b2182d8cbec6e3f863e0cfe5f395c9d)

--HG--
branch : 3.0
extra : source : 704e00560b2182d8cbec6e3f863e0cfe5f395c9d