sequencer: restrict scope of a formerly public function

sequencer: restrict scope of a formerly public function

The function to add the `exec` commands to the todo list only needed to
be public API because it was not only used internally by the sequencer,
but also by `git rebase --preserve-merges`.

Now that that mode has been removed, we no longer need that function to
be scoped publicly.

Helped-by: Alban Gruin <redacted>
Signed-off-by: Johannes Schindelin <redacted>
Reviewed-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>