diff-brief ... failed
  % 'TEST_DIFF_BRIEF=1 btest-diff child-output' failed unexpectedly (exit code 1)
  % cat .diag
  == File ===============================
  <Content not shown>
  == Diff ===============================
  @@ -1 +1 @@
  -This is the baseline
  +Hello world
  =======================================

  % cat .stderr

1 of 1 test failed
