Many projects using submodules expect all submodules to be checked out
in order to build/work correctly. A common command sequence for
developers on such projects is:

	git clone url/to/project
	cd project
	git submodule update --init (--recursive)

This patch introduces the --recursive option to git-clone. The new
option causes git-clone to recursively clone and checkout all
submodules of the cloned project. Hence, the above command sequence
can be reduced to:

	git clone --recursive url/to/project

--recursive is ignored if no checkout is done by the git-clone.

The patch also includes documentation and a selftest.
Signed-off-by: NJohan Herland <johan@herland.net>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>