While 'git checkout <submodule>' should not update the submodule's
working directory, it should update the index.  This is in line with
how submodules are handled in the rest of Git.

While at it, test 'git reset [<commit>] <submodule>', too.
Signed-off-by: NJohannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>