narrow: don't do the dirstate dance if ellipses is not enabled
I believe we set dirstate parents to nullid before widening pull because in
ellipses cases, the parent might be stripped off with a new changeset. However
the second ds.setparents() call invalidate my assumption. I am not sure why we
do this. So here is a patch.
This patch also adds tests showing we break nothing in non-ellipses cases.
Differential Revision: https://phab.mercurial-scm.org/D4788
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