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

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

Scripts working with worktree setups need a reliable way to discover
the common directory, which diverges from the git directory when
multiple worktrees are in use. There is no way to retrieve this path
from git repo info today.

Introduce path.commondir.absolute and path.commondir.relative keys.
Exposing explicit format variants rather than a single key with a
default avoids ambiguity for scripts that require predictable output.

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