submodule: fix 'submodule status' when called from a subdirectory

submodule: fix 'submodule status' when called from a subdirectory

When calling `git submodule status` while in a subdirectory, we are
incorrectly not detecting modified submodules and
thus reporting that all of the submodules are unchanged.

This is because the submodule helper is calling `diff-index` with the
submodule path assuming the path is relative to the current prefix
directory, however the submodule path used is actually relative to the root.

Always pass NULL as the `prefix` when running diff-files on the
submodule, to make sure the submodule's path is interpreted as relative
to the superproject's repository root.

Signed-off-by: Manish Goregaokar <manishsmail@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>