[m-rev.] diff: Change git submodule update command in prepare.sh.

[Julien Fischer]

On Wed, 12 Jul 2023, Peter Wang wrote:

> prepare.sh:
>    Don't pass --remote to the 'git submodule update'.
>    We want the boehm_gc submodule to be checked out at the commit
>    recorded in the mercury repository, not automatically update to the
>    latest commit on the branch listed in .gitmodules. Similarly for
>    libatomic_ops within the boehm_gc submodule.
>
>    Fix help message.

This presuambly ought to be on the release branch as well?

Julien.