setup: add support for reading extensions.objectformat

setup: add support for reading extensions.objectformat

The transition plan specifies extensions.objectFormat as the indication
that we're using a given hash in a certain repo.  Read this as one of
the extensions we support.  If the user has specified an invalid value,
fail.

Ensure that we reject the extension if the repository format version is
0.

Signed-off-by: brian m. carlson <redacted>
Reviewed-by: Eric Sunshine <redacted>
Signed-off-by: Junio C Hamano <redacted>