Oops, fixed return typo in libbe.darcs.Darcs._vcs_get_file_contents()

Oops, fixed return typo in libbe.darcs.Darcs._vcs_get_file_contents()

For self.parsed_version[0] >= 2, was returning the entire output of
._u_invoke_client() (status, output, and error).  Now it just returns
the output, which is what we want.  This fixes Chris' failed test:

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FAIL: Should get file contents as committed to specified revision.
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Traceback (most recent call last):
  File "libbe/vcs.py", line 852, in test_revision_file_contents_as_committed
    self.test_contents['rev_1'], committed_contents)
AssertionError: 'Lorem ipsum' != (0, 'Lorem ipsum', '')
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