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.

mercurial/bundle2.py

@@ -162,8 +162,6 b' from i18n import _'
 _pack = struct.pack
 _unpack = struct.unpack
 
-_magicstring = 'HG2Y'
-
 _fstreamparamsize = '>i'
 _fpartheadersize = '>i'
 _fparttypesize = '>B'
@@ -410,6 +408,8 b' class bundle20(object):'
     populate it. Then call `getchunks` to retrieve all the binary chunks of
     data that compose the bundle2 container."""
 
+    _magicstring = 'HG2Y'
+
     def __init__(self, ui, capabilities=()):
         self.ui = ui
         self._params = []
@@ -453,8 +453,8 b' class bundle20(object):'
 
     # methods used to generate the bundle2 stream
     def getchunks(self):
-        self.ui.debug('start emission of %s stream\n' % _magicstring)
-        yield _magicstring
+        self.ui.debug('start emission of %s stream\n' % self._magicstring)
+        yield self._magicstring
         param = self._paramchunk()
         self.ui.debug('bundle parameter: %s\n' % param)
         yield _pack(_fstreamparamsize, len(param))