Fix restore predefined export with more than 2 levels depth

issue3402
review1124003
(transplanted from 30b291e0bf4f17a0219b4572cee67e8fcc07acc7)

tryton/gui/window/win_export.py

@@ -359,7 +359,7 @@ class WinExport(NoModal):
                             self.on_row_expanded(self.view1, iter,
                                     self.model1.get_path(iter))
                             iter = self.model1.iter_children(iter)
-                            prefix = parent + '/'
+                            prefix += parent + '/'
                             break
                         else:
                             iter = self.model1.iter_next(iter)