Previously, commit --amend would keep the phase of the commit that's being
amended, and commit --amend --secret would force it to secret. There was no
mechanism to forcibly change a secret commit to draft in one step.
This change is being made mostly for parity with what users likely expect based
on the new behavior of commit --no-secret overriding phases.new-commit=secret;
it was confusing when writing the tests that commit --amend --no-secret didn't
operate the same.
Like with commit --no-secret, if the parent is secret, this will silently
ignore the --no-secret option and the new commit will be secret.