Unit tests for revision ids stored along with diffs.

This requires http://reviews.reviewboard.org/r/2094/ to be applied.
Before adding the tests: Ran 434 tests in 399.912s FAILED (SKIP=6, errors=3)
After adding test tests: Ran 436 tests in 354.798s FAILED (SKIP=6, errors=3)

The remaining errors are believed to be due to the tests (ungraceful handling of lacking repository support).

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ERROR: Testing a SFTP-backed bzr repository
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Traceback (most recent call last):
  File "/home/ballarin/work/reviewboard/reviewboard3/reviewboard/scmtools/tests.py", line 261, in test_sftp
    self._test_ssh(self.bzr_sftp_path, 'README')
  File "/home/ballarin/work/reviewboard/reviewboard3/reviewboard/scmtools/tests.py", line 65, in _test_ssh
    tool.check_repository(repo_path)
  File "/home/ballarin/work/reviewboard/reviewboard3/reviewboard/scmtools/bzr.py", line 221, in check_repository
    except NotBranchError, e:
NameError: global name 'NotBranchError' is not defined

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ERROR: Testing a SSH-backed bzr repository
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Traceback (most recent call last):
  File "/home/ballarin/work/reviewboard/reviewboard3/reviewboard/scmtools/tests.py", line 253, in test_ssh
    self._test_ssh(self.bzr_ssh_path, 'README')
  File "/home/ballarin/work/reviewboard/reviewboard3/reviewboard/scmtools/tests.py", line 65, in _test_ssh
    tool.check_repository(repo_path)
  File "/home/ballarin/work/reviewboard/reviewboard3/reviewboard/scmtools/bzr.py", line 221, in check_repository
    except NotBranchError, e:
NameError: global name 'NotBranchError' is not defined

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ERROR: Testing a SSH-backed bzr repository with a LocalSite
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Traceback (most recent call last):
  File "/home/ballarin/work/reviewboard/reviewboard3/reviewboard/scmtools/tests.py", line 257, in test_ssh_with_site
    self._test_ssh_with_site(self.bzr_ssh_path, 'README')
  File "/home/ballarin/work/reviewboard/reviewboard3/reviewboard/scmtools/tests.py", line 98, in _test_ssh_with_site
    lambda: tool.get_file(filename, HEAD));
  File "/usr/local/lib64/python2.6/site-packages/Django-1.3-py2.6.egg/django/utils/unittest/case.py", line 474, in assertRaises
    callableObj(*args, **kwargs)
  File "/home/ballarin/work/reviewboard/reviewboard3/reviewboard/scmtools/tests.py", line 98, in <lambda>
    lambda: tool.get_file(filename, HEAD));
  File "/home/ballarin/work/reviewboard/reviewboard3/reviewboard/scmtools/bzr.py", line 124, in get_file
    except BzrError, e:
NameError: global name 'BzrError' is not defined

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