spurious test failure in test_sha1provider_stat_and_sha1_used

[Martin Pool]

See <https://code.launchpad.net/~mbp/bzr/trivial/+merge/60735>:

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FAIL: bzrlib.tests.test__dirstate_helpers.TestUpdateEntry.test_sha1provider_stat_and_sha1_used(unicode,dirstate_Python)
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_StringException: Text attachment: log
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3201.372 creating repository in
file:///tmp/testbzr-OlVipr.tmp/bzrlib.tests.test__dirstate_helpers.TestUpdateEntry.test_sha1provider_stat_and_sha1_used%28unicode%2Cdirstate_Python%29/work/tree/.bzr/.
3201.381 creating branch <bzrlib.branch.BzrBranchFormat7 object at 0x40000210> in
file:///tmp/testbzr-OlVipr.tmp/bzrlib.tests.test__dirstate_helpers.TestUpdateEntry.test_sha1provider_stat_and_sha1_used%28unicode%2Cdirstate_Python%29/work/tree/
3201.396 trying to create missing lock
'/tmp/testbzr-OlVipr.tmp/bzrlib.tests.test__dirstate_helpers.TestUpdateEntry.test_sha1provider_stat_and_sha1_used(unicode,dirstate_Python)/work/tree/.bzr/checkout/dirstate'
3201.396 opening working tree
'/tmp/testbzr-OlVipr.tmp/bzrlib.tests.test__dirstate_helpers.TestUpdateEntry.test_sha1provider_stat_and_sha1_used(unicode,dirstate_Python)/work/tree'
3201.471 preparing to commit
    INFO Committing to: /tmp/testbzr-OlVipr.tmp/bzrlib.tests.test__dirstate_helpers.TestUpdateEntry.test_sha1provider_stat_and_sha1_used(unicode,dirstate_Python)/work/tree/
3205.646 Selecting files for commit with filter None
    INFO added a file
    INFO Committed revision 1.
3205.672 Committed revid <email address hidden> as revno 1.
3205.716 opening working tree '/tmp/testbzr-OlVipr.tmp'
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Text attachment: traceback
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Traceback (most recent call last):
  File "/usr/lib/python2.6/dist-packages/testtools/runtest.py", line 144, in _run_user
    return fn(*args)
  File "/usr/lib/python2.6/dist-packages/testtools/testcase.py", line 465, in _run_test_method
    testMethod()
  File "/home/pqm/bzr-pqm-workdir/home/+trunk/bzrlib/tests/test__dirstate_helpers.py", line 1261, in test_sha1provider_stat_and_sha1_used
    self.assertEqual(['a-file-id'], file_ids_changed)
AssertionError: not equal:
a = ['a-file-id']
b = []