repo: add the field objects.format

repo: add the field objects.format

The flag `--show-object-format` from git-rev-parse is used for
retrieving the object storage format. This way, it is used for
querying repository metadata, fitting in the purpose of git-repo-info.

Add a new field `objects.format` to the git-repo-info subcommand
containing that information.

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>