Just an obvious improvement

ERP5Diff.py

@@ -407,14 +407,14 @@ class ERP5Diff:
     old_candidate_list = old_list[:]
     new_candidate_list = new_list[:]
     for old_element in old_list:
-      old_tree = etree.fromstring(etree.tostring(old_element)).getroottree()
+      old_tree = deepcopy(old_element).getroottree()
       old_c14n = StringIO()
       old_tree.write_c14n(old_c14n)
       old_c14n.seek(0)
       for new_element in new_list:
         if new_element not in new_candidate_list:
           continue 
-        new_tree = etree.fromstring(etree.tostring(new_element)).getroottree()
+        new_tree = deepcopy(new_element).getroottree()
         new_c14n = StringIO()
         new_tree.write_c14n(new_c14n)
         new_c14n.seek(0)