demandload: implement __call__

demandload can now load functions and classes, and not just modules.

(So if you access foo() rather than just foo.* it still works).

demandload still doesn't work for constants.

mercurial/demandload.py

@@ -81,6 +81,10 b' class _replacer_from(_replacer):'
 
         return getattr(importer.module(), target)
 
+    def __call__(self, *args, **kwargs):
+        target = object.__getattribute__(self, 'module')()
+        return target(*args, **kwargs)
+
 def demandload(scope, modules):
     '''import modules into scope when each is first used.