repo: add the field references.format

repo: add the field references.format

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

The flag `--show-ref-format` from git-rev-parse is used for retrieving
the reference format (i.e. `files` or `reftable`). This way, it is
used for querying repository metadata, fitting in the purpose of
git-repo-info.

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

Helped-by: Phillip Wood <redacted>
Helped-by: Junio C Hamano <redacted>
Helped-by: Justin Tobler <redacted>
Helped-by: Eric Sunshine <redacted>
Mentored-by: Karthik Nayak <redacted>
Mentored-by: Patrick Steinhardt <redacted>
Signed-off-by: Lucas Seiki Oshiro <redacted>
Signed-off-by: Junio C Hamano <redacted>