repo: add path.git-prefix path key

repo: add path.git-prefix path key

Scripts and command-line prompt integrations frequently need to know their
relative depth inside a repository working tree layout. Currently, this
is retrieved using `git rev-parse --show-prefix`.

Introduce the `path.git-prefix` key to `git repo info`. This mirrors the
prefix location tracking framework as a standalone key, returning the
exact relative path offset complete with a trailing slash, or an empty
string if run directly at the repository working tree root.

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>