Adjust libbe.diff.DiffTree to fix failed doctest.

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FAIL: Doctest: libbe.diff.DiffTree
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Traceback (most recent call last):
  File "/usr/lib/python2.5/doctest.py", line 2128, in runTest
    raise self.failureException(self.format_failure(new.getvalue()))
AssertionError: Failed doctest test for libbe.diff.DiffTree
  File "/home/wking/src/fun/be/be.diff-subscribe/libbe/diff.py", line 136, in DiffTree

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File "/home/wking/src/fun/be/be.diff-subscribe/libbe/diff.py", line 172, in libbe.diff.DiffTree
Failed example:
    print bugdir.report_string()
Exception raised:
    Traceback (most recent call last):
      File "/usr/lib/python2.5/doctest.py", line 1228, in __run
        compileflags, 1) in test.globs
      File "<doctest libbe.diff.DiffTree[18]>", line 1, in <module>
        print bugdir.report_string()
      File "/home/wking/src/fun/be/be.diff-subscribe/libbe/diff.py", line 213, in report_string
        return "\n".join(self.report())
    TypeError

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