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 #105575
doc

00:00:48

passed #105569
flake8

00:00:44

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

00:04:17

passed #105572
tests-py2-pure

00:03:55

passed #105573
tests-py3-cext

00:03:45

passed #105574
tests-py3-pure

00:03:19