diff-max-lines ... failed
  % 'TEST_DIFF_FILE_MAX_LINES=2 btest-diff child-output' failed unexpectedly (exit code 100)
  % cat .diag
  == File ===============================
  Output line 1
  Output line 2
  Output line 3
  Output line 4
  Output line 5
  Output line 6
  Output line 7
  Output line 8
  Output line 9
  Output line 10
  == Error ==============================
  test-diff: no baseline found.
  =======================================

  % cat .stderr

1 of 1 test failed
