sha1_file: convert force_object_loose to object_id

Convert the definition and declaration of force_object_loose to
struct object_id and adjust usage of this function.

Signed-off-by: Patryk Obara <patryk.obara@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/builtin/pack-objects.c b/builtin/pack-objects.c
index 6b9cfc2..f381975 100644
--- a/builtin/pack-objects.c
+++ b/builtin/pack-objects.c
@@ -2768,7 +2768,7 @@ static void loosen_unused_packed_objects(struct rev_info *revs)
 			if (!packlist_find(&to_pack, oid.hash, NULL) &&
 			    !has_sha1_pack_kept_or_nonlocal(&oid) &&
 			    !loosened_object_can_be_discarded(&oid, p->mtime))
-				if (force_object_loose(oid.hash, p->mtime))
+				if (force_object_loose(&oid, p->mtime))
 					die("unable to force loose object");
 		}
 	}