git-submodule: clone during update, not during init

git-submodule: clone during update, not during init

This teaches 'git-submodule init' to register submodule paths and urls in
.git/config instead of actually cloning them. The cloning is now handled
as part of 'git-submodule update'.

With this change it is possible to specify preferred/alternate urls for
the submodules in .git/config before the submodules are cloned.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>