bundle2: adds a capabilities attribute on bundler20

This attribute conveys the capabilities supported by the destination of the
bundle. It is used to decide which parts to include in the bundle.

This is currently a set but will probably be turned into a dictionary to allow
capabilities with values.

mercurial/bundle2.py

@@ -340,10 +340,11 class bundle20(object):
     populate it. Then call `getchunks` to retrieve all the binary chunks of
     data that compose the bundle2 container."""
 
-    def __init__(self, ui):
+    def __init__(self, ui, capabilities=()):
         self.ui = ui
         self._params = []
         self._parts = []
+        self.capabilities = set(capabilities)
 
     def addparam(self, name, value=None):
         """add a stream level parameter"""