changegroup: add a "packermap" dictionary to track different packer versions

We only have "01" right now, but we should get general delta in soon.
Bundle2 is expected to make use of this to advertise and select the right packer
to use on both sides.

mercurial/changegroup.py

@@ -433,6 +433,8 b' class cg1packer(object):'
         # do nothing with basenode, it is implicitly the previous one in HG10
         return struct.pack(self.deltaheader, node, p1n, p2n, linknode)
 
+packermap = {'01': (cg1packer, cg1unpacker)}
+
 def _changegroupinfo(repo, nodes, source):
     if repo.ui.verbose or source == 'bundle':
         repo.ui.status(_("%d changesets found\n") % len(nodes))