hgweb: don't use mutable default argument value

mercurial/hgweb/webutil.py

@@ -142,8 +142,8 b' class filerevnav(revnav):'
         return hex(self._changelog.node(self._revlog.linkrev(rev)))
 
 class _siblings(object):
-    def __init__(self, siblings=[], hiderev=None):
-        self.siblings = [s for s in siblings if s.node() != nullid]
+    def __init__(self, siblings=None, hiderev=None):
+        self.siblings = [s for s in siblings or [] if s.node() != nullid]
         if len(self.siblings) == 1 and self.siblings[0].rev() == hiderev:
             self.siblings = []