builtin/show-index: set the algorithm for object IDs

builtin/show-index: set the algorithm for object IDs

In most cases, when we load the hash of an object into a struct
object_id, we load it using one of the oid* or *_oid_hex functions.
However, for git show-index, we read it in directly using fread.  As a
consequence, set the algorithm correctly so the objects can be used
correctly both now and in the future.

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