convert: don't use mutable default argument value

Caught by pylint.

hgext/convert/cvsps.py

@@ -622,7 +622,7 b' def createchangeset(ui, log, fuzz=60, me'
     # Sort changesets by date
 
     odd = set()
-    def cscmp(l, r, odd=odd):
+    def cscmp(l, r):
         d = sum(l.date) - sum(r.date)
         if d:
             return d