git submodule foreach: Add --recursive to recurse into nested submodules

git submodule foreach: Add --recursive to recurse into nested submodules

In very large and hierarchically structured projects, one may encounter
nested submodules. In these situations, it is valuable to not only operate
on all the submodules in the current repo (which is what is currently done
by 'git submodule foreach'), 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 foreach'
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>