tag: remove a mutable default argument

Mutable default arguments are know to the state of California to cause bugs. We
just added support of None for the underlying function, so nothing else the
required.

mercurial/localrepo.py

@@ -540,7 +540,7 b' class localrepository(object):'
         return hook.hook(self.ui, self, name, throw, **args)
 
     @unfilteredmethod
-    def _tag(self, names, node, message, local, user, date, extra={},
+    def _tag(self, names, node, message, local, user, date, extra=None,
              editor=False):
         if isinstance(names, str):
             names = (names,)