Since a078f73 (git-am: add --message-id/--no-message-id, 2014-11-25),
the am.messageid setting determines whether the --message-id option is
set by default.
Add a test for this.
Signed-off-by: Paul Tan <pyokagan@gmail.com> Reviewed-by: Paolo Bonzini <pbonzini@redhat.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>