changegroup: move revchunk() from narrow
The monkeypatched revchunk for ellipses serving is a
completely independent implementation. We model it as such
in the changegroup code. revchunk() is now a simple proxy
function.
Again, I wish we had better APIs here. Especially since this
narrow code is part of cg1packer and cg1packer can't be used
with narrow. Class inheritance is wonky. And I will definitely
be making changes to changegroup code for delta generation.
As part of the code move, `node.nullrev` was replaced by
`nullrev`. And a reference to `orig` was replaced to call
`self._revchunknormal` directly.
Differential Revision: https://phab.mercurial-scm.org/D4063