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

git submodule status: 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 show
status for all the submodules in the current repo (which is what is
currently done by 'git submodule status'), but also to show status for
all submodules at all levels (i.e. recursing into nested submodules as
well).

This patch teaches the new --recursive option to the 'git submodule status'
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>