bookmark: introduce a 'bookmarks' part

This part can carry and apply bookmarks information. We start with adding the
core behavior of the part. In its current form, the part is only suitable for
push since it plain update the bookmark without consideration for the local
state. Support of the behavior needed for pulling will be added in later
changesets.

mercurial/bundle2.py

@@ -1976,6 +1976,21 b' def handlepushkey(op, inpart):'
                 kwargs[key] = inpart.params[key]
         raise error.PushkeyFailed(partid=str(inpart.id), **kwargs)
 
+@parthandler('bookmarks')
+def handlebookmark(op, inpart):
+    """transmit bookmark information
+
+    The part contains binary encoded bookmark information. The bookmark
+    information is applied as is to the unbundling repository. Make sure a
+    'check:bookmarks' part is issued earlier to check for race condition in
+    such update.
+
+    This behavior is suitable for pushing. Semantic adjustment will be needed
+    for pull.
+    """
+    changes = bookmarks.binarydecode(inpart)
+    op.repo._bookmarks.applychanges(op.repo, op.gettransaction(), changes)
+
 @parthandler('phase-heads')
 def handlephases(op, inpart):
     """apply phases from bundle part to repo"""