builtin/maintenance: fix leak in `get_schedule_cmd()`

builtin/maintenance: fix leak in `get_schedule_cmd()`

The `get_schedule_cmd()` function allows us to override the schedule
command with a specific test command such that we can verify the
underlying logic in a platform-independent way. Its memory management is
somewhat wild though, because it basically gives up and assigns an
allocated string to the string constant output pointer. While this part
is marked with `UNLEAK()` to mask this, we also leak the local string
lists.

Rework the function such that it has a separate out parameter. If set,
we will assign it the final allocated command. Plug the other memory
leaks and create a common exit path.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>