[PATCH 1 of 2 V2] util: add a way to issue deprecation warning without a UI object

[Jun Wu]

Excerpts from Yuya Nishihara's message of 2017-04-06 22:58:24 +0900:
> If dirty hack allowed, I would do something like the following:
> 
>   # util.py
>   def _deprecwarn(msg, version):
>       pass
> 
>   # somewhere ui is available, maybe in dispatch.py
>   util._deprecwarn = ui.deprecwarn

I think this is actually better since it works better with chg -
util._deprecwarn could be replaced without restarting.