changegroup: specify ellipses mode explicitly Currently, code throughout changegroup relies on the presence of self._full_nodes to enable ellipses mode. This is a very tenuous check. And the check may be wrong once we move _full_nodes into cgpacker. Let's capture the enabling of ellipses mode explicitly as a constructor argument and as an instance variable. We could probably derive ellipses mode by presence of other variables. But for now, this explicit approach seems simplest since it is most compatible with existing code. Differential Revision: https://phab.mercurial-scm.org/D4090