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>

command-list.txt

@@ -164,6 +164,7 @@ git-remote                              ancillarymanipulators           complete
 git-repack                              ancillarymanipulators           complete
 git-replace                             ancillarymanipulators           complete
 git-replay                              plumbingmanipulators
+git-repo                                plumbinginterrogators
 git-request-pull                        foreignscminterface             complete
 git-rerere                              ancillaryinterrogators
 git-reset                               mainporcelain           history