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.
8 jobs for topic/default/evolve-dirstate-parents in 7 minutes and 5 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed checks-py2 #111075

00:02:15

passed checks-py3 #111076

00:02:30

passed doc #111081

00:00:40

manual pytype #111074
allowed to fail manual
passed tests-py2-cext #111077

00:04:42

passed tests-py2-pure #111078

00:04:40

passed tests-py3-cext #111079

00:05:23

passed tests-py3-pure #111080

00:04:07