changelog: add the missing 'closed' property on 'appender' object

This mimic file object further.

mercurial/changelog.py

@@ -90,6 +90,11 b' class appender(object):'
         return self.offset
     def flush(self):
         pass
+
+    @property
+    def closed(self):
+        return self.fp.closed
+
     def close(self):
         self.fp.close()