add update 'none' flag to disable update of submodule by default
This is useful to mark a submodule as unneeded by default. When this
option is set and the user wants to work with such a submodule he
needs to configure 'submodule.<name>.update=checkout' or pass the
--checkout option. Then the submodule can be handled like a normal
submodule.
Signed-off-by: Heiko Voigt <hvoigt@hvoigt.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>