templater: remove a mutable default argument

Mutable default arguments are know to the state of California to cause bugs.

mercurial/templater.py

@@ -792,8 +792,10 class engine(object):
     filter uses function to transform value. syntax is
     {key|filter1|filter2|...}.'''
 
-    def __init__(self, loader, filters={}, defaults={}):
+    def __init__(self, loader, filters=None, defaults={}):
         self._loader = loader
+        if filters is None:
+            filters = {}
         self._filters = filters
         self._defaults = defaults
         self._cache = {}