loose: add a mapping between SHA-1 and SHA-256 for loose objects

loose: add a mapping between SHA-1 and SHA-256 for loose objects

As part of the transition plan, we'd like to add a file in the .git
directory that maps loose objects between SHA-1 and SHA-256.  Let's
implement the specification in the transition plan and store this data
on a per-repository basis in struct repository.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Eric W. Biederman <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>