commit-graph: don't store file hashes as struct object_id

commit-graph: don't store file hashes as struct object_id

The idea behind struct object_id is that it is supposed to represent the
identifier of a standard Git object or a special pseudo-object like the
all-zeros object ID.  In this case, we have file hashes, which, while
similar, are distinct from the identifiers of objects.

Switch these code paths to use an unsigned char array.  This is both
more logically consistent and it means that we need not set the
algorithm identifier for the struct object_id.

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