combine_details blows up when there are duplicate details

[Jonathan Lange]

combine_details refers to advance_iterator, which is an unknown name. This means that whenever getDetails is called for duplicate details (say, a parent with 'traceback' and a child with 'traceback'), then combine_details will raise an error that looks like this:

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ERROR: fixtures.tests.test_fixture.TestFixture.test_duplicate_details_are_disambiguated
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Traceback (most recent call last):
  File "/home/jml/src/python-fixtures/busted-combine-details/lib/fixtures/tests/test_fixture.py", line 176, in test_duplicate_details_are_disambiguated
    'foo-1': 'child-content',}, parent.getDetails())
  File "/home/jml/src/python-fixtures/busted-combine-details/lib/fixtures/fixture.py", line 156, in getDetails
    combine_details(source.getDetails(), result)
  File "/home/jml/src/python-fixtures/busted-combine-details/lib/fixtures/fixture.py", line 44, in combine_details
    new_name = '%s-%d' % (name, advance_iterator(disambiguator))
NameError: global name 'advance_iterator' is not defined

I believe this to be a Critical bug.