RangeSet: fix performance regression in difference_update (#346)

Optimize RangeSet.difference_update() by avoiding object copy and
also possible costly recursion in case of KeyError in strict mode.

Fixes #346.

Change-Id: Ic35a26ac40e350d8fe8b8108766a0126895b245b

lib/ClusterShell/RangeSet.py

@@ -688,7 +688,7 @@ def difference_update(self, other, strict=False):
         If strict is True, raise KeyError if an element cannot be removed.
         (strict is a RangeSet addition)"""
         if strict and other not in self:
-            raise KeyError(other.difference(self)[0])
+            raise KeyError(set.difference(other, self).pop())
         set.difference_update(self, other)
 
     # Python dict-like mass mutations: update, clear