We have had multiple bugs where the phase wasn't correctly carried
forward to a rewritten changeset (for example: phabricator, split,
evolve, fix). Handling the phase update in cleanupnodes() makes it
less likely to happen again, especially once we have made it fix the
phase by default (perhaps in the next release cycle).
This patch also updates all applicable callers so we get some testing
Note that rebase and histedit can't be fixed yet because they call
cleanupnodes() only at the end and the phase may have been changed by
the user when the rebase/histedit was interrupted (due to merge
conflicts). I think we should make them write one commit at a time (as
it already does), along with associated obsmarkers, bookmark moves,
etc. When that's done, we can switch them over to cleanupnodes().