trial --temp-directory= blows up

New submission from ChrisW <None>:

In an effort to stop trial dropping spoor all over my svn checkouts, I tried:

$ mkdir /tmp/_trial_temp
$ trial --temp-directory=/tmp/_trial_temp
Traceback (most recent call last):
  File "/usr/bin/trial", line 5, in <module>
    pkg_resources.run_script('Twisted==9.0.0', 'trial')
  File "/usr/lib/python2.5/site-packages/pkg_resources.py", line 448, in run_script
    self.require(requires)[0].run_script(script_name, ns)
  File "/usr/lib/python2.5/site-packages/pkg_resources.py", line 1166, in run_script
    execfile(script_filename, namespace, namespace)
  File "/usr/lib/python2.5/site-packages/Twisted-9.0.0-py2.5-linux-x86_64.egg/EGG-INFO/scripts/trial", line 22, in <module>
    run()
  File "/usr/lib/python2.5/site-packages/Twisted-9.0.0-py2.5-linux-x86_64.egg/twisted/scripts/trial.py", line 363, in run
    test_result = trialRunner.run(suite)
  File "/usr/lib/python2.5/site-packages/Twisted-9.0.0-py2.5-linux-x86_64.egg/twisted/trial/runner.py", line 833, in run
    return self._runWithoutDecoration(test)
  File "/usr/lib/python2.5/site-packages/Twisted-9.0.0-py2.5-linux-x86_64.egg/twisted/trial/runner.py", line 859, in _runWithoutDecoration
    oldDir = self._setUpTestdir()
  File "/usr/lib/python2.5/site-packages/Twisted-9.0.0-py2.5-linux-x86_64.egg/twisted/trial/runner.py", line 753, in _setUpTestdir
    self._removeSafely(testdir)
  File "/usr/lib/python2.5/site-packages/Twisted-9.0.0-py2.5-linux-x86_64.egg/twisted/trial/runner.py", line 720, in _removeSafely
    % (path,))
twisted.trial.runner._NoTrialMarker: FilePath('/tmp/_trial_temp') is not a trial temporary path, refusing to remove it


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Type     : defect
Component: trial
Keywords : 
Priority : normal
Nosy     : 
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http://twistedmatrix.com/trac/ticket/4215