mq: remove a mutable default argument

Mutable default arguments are know to the state of California to cause bugs. The
underlying function already handle "None" as an option value, so we do not need
to do anything else.

hgext/mq.py

@@ -483,7 +483,7 class queue(object):
         self.guardsdirty = False
         self.activeguards = None
 
-    def diffopts(self, opts={}, patchfn=None):
+    def diffopts(self, opts=None, patchfn=None):
         diffopts = patchmod.diffopts(self.ui, opts)
         if self.gitmode == 'auto':
             diffopts.upgrade = True