With this change, hg split will preserve the phase of the commit that is being
split, ignoring the phases.new-commit setting. Previously, we would use whatever
phases.new-commit was set to (unless our parent was secret, then we would be
secret even if phases.new-commit=draft).
Now, splitting a draft commit with phases.new-commit=secret does not cause the
new commits to become secret, and splitting a secret commit with
phases.new-commit=draft and a draft parent does not cause the new commits to
Test cases and commit message taken from Kyle Lippincott's D2016 (thanks!).