rev-parse: add --show-superproject-working-tree

rev-parse: add --show-superproject-working-tree

In some situations it is useful to know if the given repository
is a submodule of another repository.

Add the flag --show-superproject-working-tree to git-rev-parse
to make it easy to find out if there is a superproject. When no
superproject exists, the output will be empty.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>