t7003: compute appropriate length constant

Instead of using a specific invalid hard-coded object ID, look one
up from the translation table.

Signed-off-by: brian m. carlson <[email protected]>
Reviewed-by: Eric Sunshine <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: bk2204

t/t7003-filter-branch.sh

@@ -463,10 +463,11 @@ test_expect_success 'rewrite submodule with another content' '
 '
 
 test_expect_success 'replace submodule revision' '
+	invalid=$(test_oid numeric) &&
 	git reset --hard original &&
 	git filter-branch -f --tree-filter \
 	    "if git ls-files --error-unmatch -- submod > /dev/null 2>&1
-	     then git update-index --cacheinfo 160000 0123456789012345678901234567890123456789 submod
+	     then git update-index --cacheinfo 160000 $invalid submod
 	     fi" HEAD &&
 	test $orig_head != $(git show-ref --hash --head HEAD)
 '