Cannot disable diffs - err.showDiff not working

[vjpr]

When err.showDiff is false it does not prevent a diff being shown.

Here is the only place where showDiff is being used in Mocha:

    // msg
    var err = test.err
      , message = err.message || ''
      , stack = err.stack || message
      , index = stack.indexOf(message) + message.length
      , msg = stack.slice(0, index)
      , actual = err.actual
      , expected = err.expected
      , escape = true;

    // explicitly show diff
    if (err.showDiff) {
      escape = false;
      err.actual = actual = JSON.stringify(actual, null, 2);
      err.expected = expected = JSON.stringify(expected, null, 2);
    }

I think its just been overlooked.