Add --depth to submodule update/add

Add --depth to submodule update/add

Add the --depth option to the add and update commands of "git submodule",
which is then passed on to the clone command. This is useful when the
submodule(s) are huge and you're not really interested in anything but
the latest commit.

Tests are added and some indention adjustments were made to conform to the
rest of the testfile on "submodule update can handle symbolic links in pwd".

Signed-off-by: Fredrik Gustafsson <iveqy@iveqy.com>
Acked-by: Jens Lehmann <Jens.Lehmann@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>