t/lib-submodule-update: use appropriate length constant

Instead of using a specific invalid hard-coded object ID, produce one
of the appropriate length by using test_oid.

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

t/lib-submodule-update.sh

@@ -139,7 +139,7 @@ create_lib_submodule_repo () {
 		git revert HEAD &&
 
 		git checkout -b invalid_sub1 add_sub1 &&
-		git update-index --cacheinfo 160000 0123456789012345678901234567890123456789 sub1 &&
+		git update-index --cacheinfo 160000 $(test_oid numeric) sub1 &&
 		git commit -m "Invalid sub1 commit" &&
 		git checkout -b valid_sub1 &&
 		git revert HEAD &&
@@ -196,6 +196,7 @@ test_git_directory_exists() {
 # the submodule repo if it doesn't exist and configures the most problematic
 # settings for diff.ignoreSubmodules.
 prolog () {
+	test_oid_init &&
 	(test -d submodule_update_repo || create_lib_submodule_repo) &&
 	test_config_global diff.ignoreSubmodules all &&
 	test_config diff.ignoreSubmodules all