evolve: don't set evolved node as dirstate p2
Upstream rebase changed in 9c9cfecd4600 (rebase: don't use rebased node as dirstate p2 (BC), 2020-01-10) so the dirstate parents are always the commits that will be parents once the commit has been created. Let's follow their lead so the experience is consistent. The drawback is that users who use Evolve with older Mercurial versions will not see in `hg log -G` output which commit is being evolved.
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