Some power user workflow requires uncommit to leave an empty commit so
they can run hg amend afterwards. I don't find easy alternatives so let's
allow that if ui.allowemptyconfig is set.
Note: ui.allowemptyconfig is an internal config that is not documented for
end-users. Core Mercurial seems to be trying to prevent the empty commit
case from normal users (like, comment disallows that, rebase will skip a
commit if after rebasing it becomes empty). So it seems better to still use
that undocumented config instead of a more discoverable flag.