repo: add path.gitdir with absolute and relative suffix formatting

repo: add path.gitdir with absolute and relative suffix formatting

Scripts need a stable way to locate the git directory without
parsing rev-parse output or relying on its flag-driven path format
selection. There is no way to retrieve this path from git repo info
today.

Introduce path.gitdir.absolute and path.gitdir.relative keys,
consistent with the path.commondir keys added in the previous patch.
Reuse the test_repo_info_path helper introduced there to validate
both variants.

Mentored-by: Justin Tobler <redacted>
Mentored-by: Lucas Seiki Oshiro <redacted>
Signed-off-by: K Jayatheerth <redacted>
Signed-off-by: Junio C Hamano <redacted>