repo: add the field layout.shallow

repo: add the field layout.shallow

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

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

Then, add a new field `layout.shallow` to the git-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>