[FIX] purchase: keep track of all procurements

Keep track of all the procurements in the field "Source Document" of the
PO.

opw-654424

addons/purchase/purchase.py

@@ -822,6 +822,12 @@ class ProcurementOrder(models.Model):
                 vals = procurement._prepare_purchase_order(partner)
                 po = self.env['purchase.order'].create(vals)
                 cache[domain] = po
+            elif not po.origin or procurement.origin not in po.origin.split(', '):
+                # Keep track of all procurements
+                if po.origin:
+                    po.write({'origin': po.origin + ', ' + procurement.origin})
+                else:
+                    po.write({'origin': procurement.origin})
             res += po.ids
 
             # Create Line