tests: make test_must_fail more verbose

diff --git a/t/t4020-diff-external.sh b/t/t4020-diff-external.sh
index 4ea42e00dad9bf15e221e6eac5d91748cb0a07e7..a7602cf923d95a51643753ce44fa65cf406aba9c 100755 (executable)
--- a/t/t4020-diff-external.sh
+++ b/t/t4020-diff-external.sh
@@ -166,7 +166,7 @@ test_expect_success 'diff --cached' '
        git update-index --assume-unchanged file &&
        echo second >file &&
        git diff --cached >actual &&
-       test_cmp ../t4020/diff.NUL actual
+       test_cmp "$TEST_DIRECTORY"/t4020/diff.NUL actual
 '
 
 test_done