Fix completion test for Reference field

The test must use only the id and should also allow negative id.

issue3700
review3691003

tryton/gui/window/view_form/view/form_gtk/many2one.py

@@ -380,7 +380,8 @@ class Many2One(WidgetInterface):
             return
         if not self.record:
             return
-        if self.field.get(self.record) is not None:
+        id_ = self.id_from_value(self.field.get(self.record))
+        if id_ is not None and id_ >= 0:
             return
         model = self.get_model()
         update_completion(self.wid_text, self.record, self.field, model)