bundle2: introduce a `params` dictionary on unbundled parts

This exposes all parameters the part received into a ``part.params`` dictionary.
This should be much easier to use.

This dictionary itself does not expose the mandatory or advisory aspect of
parameters, but no current users of bundle2 actually enforce any of this logic.
Coming changesets will improve this aspect.

mercurial/bundle2.py

@@ -689,6 +689,7 @@ class unbundlepart(unpackermixin):
         self.type = None
         self.mandatoryparams = None
         self.advisoryparams = None
+        self.params = None
         self._payloadstream = None
         self._readheader()
 
@@ -711,6 +712,9 @@ class unbundlepart(unpackermixin):
         # make it read only to prevent people touching it by mistake.
         self.mandatoryparams = tuple(mandatoryparams)
         self.advisoryparams  = tuple(advisoryparams)
+        # user friendly UI
+        self.params = dict(self.mandatoryparams)
+        self.params.update(dict(self.advisoryparams))
 
     def _readheader(self):
         """read the header and setup the object"""