changegroup: populate _clnodetorev as part of changelog linknode lookup
The thing that matters is that self._clnodetorev is populated with
changesets that are being sent. Back when this code was in an
extension, it wasn't possible to monkeypatch the changelog lookup
function. Now that the code is in core, we can move this code to
where it logically belongs.
Differential Revision: https://phab.mercurial-scm.org/D4186
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