Convert struct object to object_id

struct object is one of the major data structures dealing with object
IDs.  Convert it to use struct object_id instead of an unsigned char
array.  Convert get_object_hash to refer to the new member as well.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Jeff King <peff@peff.net>

object.c

@@ -168,7 +168,7 @@ void *object_as_type(struct object *obj, enum object_type type, int quiet)
 	else {
 		if (!quiet)
 			error("object %s is a %s, not a %s",
-			      sha1_to_hex(obj->sha1),
+			      oid_to_hex(&obj->oid),
 			      typename(obj->type), typename(type));
 		return NULL;
 	}