loose: compatibilty short name support

loose: compatibilty short name support

Update loose_objects_cache when udpating the loose objects map.  This
oidtree is used to discover which oids are possibilities when
resolving short names, and it can support a mixture of sha1
and sha256 oids.

With this any oid recorded objects/loose-objects-idx is usable
for resolving an oid to an object.

To make this maintainable a helper insert_loose_map is factored
out of load_one_loose_object_map and repo_add_loose_object_map,
and then modified to also update the loose_objects_cache.

Signed-off-by: "Eric W. Biederman" <ebiederm@xmission.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>