submodule.c: stricter checking for submodules in is_submodule_modified

By having a stricter check in the superproject we catch errors earlier,
instead of spawning a child process to tell us.

Signed-off-by: Stefan Beller <sbeller@google.com>
Reviewed-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>