bundle2: add a 'modes' dictionary to the bundle operation

This new attribute allows the codes requesting an unbundling to pass important
information to individual part handlers. The current target use case is to
allow for receiving 'bookmarks' part without directly updating local
repository, but just recording the received data instead. This is necessary
for pull where the remote bookmarks are processed locally. I expect the
concept to be beneficial to other parts in the future.

To clarify the bookmark behavior on pull, the remote bookmark value are not just
taken -as-is- into the local repository. There is an extra step to detect
bookmark divergence. The remote bookmarks data are stored until this processing
happens.

mercurial/bundle2.py

@@ -301,6 +301,8 b' class bundleoperation(object):'
         self.captureoutput = captureoutput
         self.hookargs = {}
         self._gettransaction = transactiongetter
+        # carries value that can modify part behavior
+        self.modes = {}
 
     def gettransaction(self):
         transaction = self._gettransaction()