bundle20: move magic string into the class...
bundle20: move magic string into the class
This makes it easy to create a new bundler class that inherits from
the core one. This matches the way 'changegroup' packers work.
The main target is to allow HG2Y support in an extension to ease transition of
companies using the experimental protocol in production (yeah...) But I've no
doubt this will be useful when playing with a future HG21.