bundle2: make sure unbundled part param are read-only

My old uncle Robert once trusted an API user. We never saw him again.

mercurial/bundle2.py

@@ -708,8 +708,9 b' class unbundlepart(unpackermixin):'
 
     def _initparams(self, mandatoryparams, advisoryparams):
         """internal function to setup all logic related parameters"""
-        self.mandatoryparams = mandatoryparams
-        self.advisoryparams  = advisoryparams
+        # make it read only to prevent people touching it by mistake.
+        self.mandatoryparams = tuple(mandatoryparams)
+        self.advisoryparams  = tuple(advisoryparams)
 
     def _readheader(self):
         """read the header and setup the object"""