submodule: fix status of initialized but not cloned submodules

Original bash helper for "submodule status" was doing a check for
initialized but not cloned submodules and prefixed the status with
a minus sign in case no .git file or folder was found inside the
submodule directory.

This check was missed when the original port of the functionality
from bash to C was done.

Signed-off-by: Peter Kaestle <peter.kaestle@nokia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
