changegroup: note why a few methods on cg1unpacker exist

I'm not sure what to do abstraction-wise here. It might be more
sensible to make a memoryrepo that could apply a bundle in-memory and
then we could make the changegroup data be strictly an applyable
stream, but that's an idea for Later.

mercurial/changegroup.py

@@ -182,6 +182,9 b' class cg1unpacker(object):'
         self._stream = util.decompressors[alg](fh)
         self._type = alg
         self.callback = None
+
+    # These methods (compressed, read, seek, tell) all appear to only
+    # be used by bundlerepo, but it's a little hard to tell.
     def compressed(self):
         return self._type is not None
     def read(self, l):