unbundle20: allow registering handlers for stream level parameters

As a comment in the code have been asking for, it is now possible to register
piece of code that handle parameters for the stream. I've been wondering is such
function should be class methods or not. I eventually went for externally
decorated methods to stick with the culture of extensibility from an extensions
that apply to bundle2.

mercurial/bundle2.py

@@ -661,13 +661,15 b' class unbundle20(unpackermixin):'
             raise ValueError('empty parameter name')
         if name[0] not in string.letters:
             raise ValueError('non letter first character: %r' % name)
-        # Some logic will be later added here to try to process the option for
-        # a dict of known parameter.
+        try:
+            handler = b2streamparamsmap[name.lower()]
+        except KeyError:
         if name[0].islower():
             indebug(self.ui, "ignoring unknown parameter %r" % name)
         else:
             raise error.BundleUnknownFeatureError(params=(name,))
-
+        else:
+            handler(self, name, value)
 
     def iterparts(self):
         """yield all parts contained in the stream"""
@@ -700,6 +702,16 b' class unbundle20(unpackermixin):'
 
 formatmap = {'20': unbundle20}
 
+b2streamparamsmap = {}
+
+def b2streamparamhandler(name):
+    """register a handler for a stream level parameter"""
+    def decorator(func):
+        assert name not in formatmap
+        b2streamparamsmap[name] = func
+        return func
+    return decorator
+
 class bundlepart(object):
     """A bundle2 part contains application level payload