support __getitem__ with negative value except -1.

git-svn-id: https://svn.erp5.org/repos/public/erp5/trunk@18539 20353a03-c40f-0410-a6d1-a30d3c3de9de

product/ERP5Type/patches/WorkflowTool.py

@@ -631,6 +631,9 @@ class WorkflowHistoryList(Persistent):
         if index == -1:
             return self._slots[-1]
         elif isinstance(index, (int, long)):
+            if index < 0:
+                # XXX this implementation is not so good, but rarely used.
+                index += len(self)
             iterator = self.__iter__()
             for i in xrange(index):
                 iterator.next()