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
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