templatefilters.json: call functions

The "changeset" template from hgweb is using a lambda in the
"diffsummary" key. In preparation for enabling JSON output from hgweb,
teach the json filter how to call functions.

mercurial/templatefilters.py

@@ -208,6 +208,8 b' def json(obj):'
         for i in obj:
             out.append(json(i))
         return '[' + ', '.join(out) + ']'
+    elif util.safehasattr(obj, '__call__'):
+        return json(obj())
     else:
         raise TypeError('cannot encode type %s' % obj.__class__.__name__)