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