[FIX] purchase: Wrong scheduled date for backorder

Steps to reproduce:
-Create a PO with two lines L1 and L2
-L1 has product P with qty=10 for a scheduled date=23/07/2017
-L2 has product P with qty=10 for a scheduled date=30/07/2017
-Confirm the PO

When delivering 5P of L1, the scheduled date of the picking is the
right date which is 23/07/2017
But when delivering the other 5P of L1, the scheduled date of the
second picking was 30/07/2017 instead of 23/07/2017.

opw:752192

addons/purchase/purchase.py

@@ -357,6 +357,10 @@ class PurchaseOrder(models.Model):
                 picking = self.env['stock.picking'].create(res)
                 moves = order.order_line.filtered(lambda r: r.product_id.type in ['product', 'consu'])._create_stock_moves(picking)
                 move_ids = moves.action_confirm()
+                seq = 0
+                for move in moves:
+                    seq += 5
+                    move.sequence = seq
                 moves = self.env['stock.move'].browse(move_ids)
                 moves.force_assign()
         return True