Fails to build with diffutils 3.5

diffutils 1:3.5-1 in Debian unstable includes the following change:

  * Exit status of diff for binary files that differ is now 1, not 2,
    as mandated by POSIX. Closes: #737180.

This causes the Bazaar test suite to fail:

======================================================================
FAIL: bzrlib.tests.test_diff.TestDiffFiles.test_external_diff_binary
----------------------------------------------------------------------
Traceback (most recent call last):
testtools.testresult.real._StringException: Empty attachments:
  log

Traceback (most recent call last):
  File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/tests/test_diff.py", line 317, in test_external_diff_binary
    self.assertEqual(2, pipe.returncode)
  File "/<<BUILDDIR>>/bzr-2.7.0+bzr6619/build/lib.linux-x86_64-2.7/bzrlib/tests/__init__.py", line 1370, in assertEqual
    pprint.pformat(a), pprint.pformat(b)))
AssertionError: not equal:
a = 2
b = 1