repo: add the field layout.bare

This commit is part of the series that introduces the new subcommand
git-repo-info.

The flag --is-bare-repository from git-rev-parse is used for retrieving
whether the current repository is bare. This way, it is used for
querying repository metadata, fitting in the purpose of git-repo-info.

Then, add a new field layout.bare to the git-repo-info subcommand
containing that information.

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>

Documentation/git-repo.adoc

@@ -38,6 +38,9 @@ In order to obtain a set of values from `git repo info`, you should provide
 the keys that identify them. Here's a list of the available keys and the
 values that they return:
 
+`layout.bare`::
+	`true` if this is a bare repository, otherwise `false`.
+
 `references.format`::
 	The reference storage format. The valid values are:
 +