InitTestCase.test_clone_with_refs fails on bionic

When testing a redeployment on bionic, I ran into this:

FAIL: test_clone_with_refs (api.tests.test_store.InitTestCase)
api.tests.test_store.InitTestCase.test_clone_with_refs
----------------------------------------------------------------------
_StringException: Traceback (most recent call last):
  File "/srv/turnip/turnip-pack-backend/payloads/e7af7a12a8592d4f1f6e48231a86cac22e2e8fe6/turnip/api/tests/test_store.py", line 168, in test_clone_with_refs
    [], to_path)
  File "/srv/turnip/turnip-pack-backend/payloads/e7af7a12a8592d4f1f6e48231a86cac22e2e8fe6/turnip/api/tests/test_store.py", line 54, in assertAdvertisedRefs
    hex.encode('ascii') + b' ' + ref.encode('utf-8'), out)
  File "/srv/turnip/turnip-pack-backend/payloads/e7af7a12a8592d4f1f6e48231a86cac22e2e8fe6/env/local/lib/python2.7/site-packages/testtools/testcase.py", line 356, in assertIn
    self.assertThat(haystack, Contains(needle), message)
  File "/srv/turnip/turnip-pack-backend/payloads/e7af7a12a8592d4f1f6e48231a86cac22e2e8fe6/env/local/lib/python2.7/site-packages/testtools/testcase.py", line 435, in assertThat
    raise mismatch_error
testtools.matchers._impl.MismatchError: '2124b7dd8da3532a8f2989f1c253a7b66bfed00c .have' not in '00910a2a57e739d17f242653bed3076ab41a260fc43a refs/heads/branch-0\x00report-status delete-refs side-band-64k quiet atomic ofs-delta agent=git/2.17.1\n0041a24c5718c7ed76af85125051280aa698f46d4d69 refs/heads/branch-1\n00412124b7dd8da3532a8f2989f1c253a7b66bfed00c refs/heads/branch-2\n003f2124b7dd8da3532a8f2989f1c253a7b66bfed00c refs/heads/master\n0000'

I bisected this to this commit from git 2.13.0:

  https://github.com/git/git/commit/63d428e656edcd670fa87e74136726096ff3de6f

Based on that commit message and the test code, I think we probably just need to rearrange the test a bit so that we aren't testing for the presence of a ".have" that's also in the main repository, but I haven't checked that thoroughly yet.