repo: declare the repo command

repo: declare the repo command

Currently, `git rev-parse` covers a wide range of functionality not
directly related to parsing revisions, as its name suggests. Over time,
many features like parsing datestrings, options, paths, and others
were added to it because there wasn't a more appropriate command
to place them.

Create a new Git command called `repo`. `git repo` will be the main
command for obtaining the information about a repository (such as
metadata and metrics).

Also declare a subcommand for `repo` called `info`. `git repo info`
will bring the functionality of retrieving repository-related
information currently returned by `rev-parse`.

Add the required documentation and build changes to enable usage of
this subcommand.

Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Helped-by: Junio C Hamano <gitster@pobox.com>
Helped-by: Justin Tobler <jltobler@gmail.com>
Helped-by: Eric Sunshine <sunshine@sunshineco.com>
Mentored-by: Karthik Nayak <karthik.188@gmail.com>
Mentored-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Lucas Seiki Oshiro <lucasseikioshiro@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>