fold: when folding drafts, make result draft even if phases.new-commit=secret

Since hg 4.7, there has been a `fixphase` argument to
`scmutil.cleanupnodes()`. This patch makes it so we use that for `hg
fold` if it's available. I'll leave it for later to pass it for all
the other commands.

--HG--
branch : stable
7 jobs for topic/stable/fold-phases in 4 minutes and 17 seconds (queued for 1 second)
Status Job ID Name Coverage
  Test
passed doc #105575

00:00:48

passed flake8 #105569

00:00:44

manual pytype #105570
allowed to fail manual
passed tests-py2-cext #105571

00:04:17

passed tests-py2-pure #105572

00:03:55

passed tests-py3-cext #105573

00:03:45

passed tests-py3-pure #105574

00:03:19