add--interactive: ignore submodule changes except HEAD

add--interactive: ignore submodule changes except HEAD

For 'add -i' and 'add -p', the only action we can take on a dirty
submodule entry is update the index with a new value from its HEAD. The
content changes inside (from its own index, untracked files...) do not
matter, at least until 'git add -i' learns about launching a new
interactive add session inside a submodule.

Ignore all other submodules changes except HEAD. This reduces the number
of entries the user has to check through in 'git add -i', and the number
of 'no' they have to answer to 'git add -p' when dirty submodules are
present.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>