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.
8 jobs for topic/default/fold-phases in 4 minutes and 27 seconds (queued for 1 second)