repo: add path.superproject-working-tree with absolute and relative suffixes

repo: add path.superproject-working-tree with absolute and relative suffixes

Scripts working in multi-repository setups often need to identify the
top-level working tree of a superproject from within a submodule.
Currently, this is only exposed via `git rev-parse
--show-superproject-working-tree`.

Introduce `path.superproject-working-tree.absolute` and
`path.superproject-working-tree.relative` keys to `git repo info`.
This exposes the core submodule context via a scriptable config-like key
using standard format rules.

If requested when not inside a submodule, the command returns an empty
string.

Mentored-by: Justin Tobler <jltobler@gmail.com>
Mentored-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: K Jayatheerth <jayatheerthkulkarni2005@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>