[FIX] sale: _get_display_price with pricelist item based on other pricelist

Create a product P with a price = 1799
Make a pricelist P1 in USD with an item I1 for a product P with a
fixed price to 100.
Set P1 with discount_policy = "with_discount"(include in price)
Make another pricelist P2 in USD with an item I2 based on P1.
I2 is a formula = 50% of the price
Set P2 with discount_policy = "without_discount"
Create a SO with P2 for the product P

Bug: The subtotal of P is 899.5 instead of 50

The problem came from _get_display_price wich in this case recomputed
the price according to the discount(in this case equal to 50% and this is
the good discount to display because P1 include the discount) and the lst_price
of the product(=1799) So the displayed price was equal to 899.5 instead of
50.

All the other cases worked because

1.  P1 without_discount and P2 without_discount:

Price returned by _get_display_price was equal to 1799 with a discount of 97,2%
So the subtotal was equal to 50

2. P1 without_discount and P2 with_discount:

Price returned by _get_display_price was equal to 100 with a discount of 50%
So the subtotal was equal to 50

3. P1 with_discount and P2 with_discount:

Price returned by _get_display_price was equal to 50 with a discount of 0%
So the subtotal was equal to 50

And the last case(P1 with_discount and P2 without_discount) didn't work because:

Price returned by _get_display_price was equal to 1799 with a discount of 50%
So the subtotal was equal to 899.5

NB: The subtotal is computed with function "_compute_amount" with the following line:

price = line.price_unit * (1 - (line.discount or 0.0) / 100.0)

opw:724924

addons/sale/models/sale.py

@@ -835,10 +835,16 @@ class SaleOrderLine(models.Model):
     @api.multi
     def _get_display_price(self, product):
         # TO DO: move me in master/saas-16 on sale.order
-        if self.order_id.pricelist_id.discount_policy == 'without_discount':
+        if self.order_id.pricelist_id.discount_policy == 'with_discount':
+            return product.with_context(pricelist=self.order_id.pricelist_id.id).price
+        price, rule_id = self.order_id.pricelist_id.get_product_price_rule(self.product_id, self.product_uom_qty or 1.0, self.order_id.partner_id)
+        pricelist_item = self.env['product.pricelist.item'].browse(rule_id)
+        if (pricelist_item.base == 'pricelist' and pricelist_item.base_pricelist_id.discount_policy == 'with_discount'):
+            price, rule_id = pricelist_item.base_pricelist_id.get_product_price_rule(self.product_id, self.product_uom_qty or 1.0, self.order_id.partner_id)
+            return price
+        else:
             from_currency = self.order_id.company_id.currency_id
             return from_currency.compute(product.lst_price, self.order_id.pricelist_id.currency_id)
-        return product.with_context(pricelist=self.order_id.pricelist_id.id).price
 
     @api.multi
     @api.onchange('product_id')