git submodule update: Introduce --recursive to update nested submodules

git submodule update: Introduce --recursive to update nested submodules

In very large and hierarchically structured projects, one may encounter
nested submodules. In these situations, it is valuable to not only update
the submodules in the current repo (which is what is currently done by
'git submodule update'), but also to operate on all submodules at all
levels (i.e. recursing into nested submodules as well).

This patch teaches the new --recursive option to the 'git submodule update'
command. The patch also includes documentation and selftests.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>