Generating merge diff fails to resolve delete/delete conflict

[Colin Watson]

We ran into OOPS-18c97ad3b730e2b3f26dcf2cd39a4638 while generating a preview diff for a merge proposal: the situation was that some files had been renamed in the source branch, and then that renaming and some related changes were split out to a separate merge proposal and landed separately. The traceback on the turnip side was:

Traceback (most recent call last):
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/gunicorn/workers/sync.py", line 130, in handle
    self.handle_request(listener, req, client, addr)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/gunicorn/workers/sync.py", line 171, in handle_request
    respiter = self.wsgi(environ, resp.start_response)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/pyramid/router.py", line 242, in __call__
    response = self.invoke_subrequest(request, use_tweens=True)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/pyramid/router.py", line 217, in invoke_subrequest
    response = handle_request(request)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/pyramid/tweens.py", line 46, in excview_tween
    response = view_callable(exc, request)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/pyramid/config/views.py", line 385, in viewresult_to_response
    result = view(context, request)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/pyramid/tweens.py", line 21, in excview_tween
    response = handler(request)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/pyramid/router.py", line 163, in handle_request
    response = view_callable(context, request)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/pyramid/config/views.py", line 596, in __call__
    return view(context, request)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/pyramid/config/views.py", line 329, in attr_view
    return view(context, request)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/pyramid/config/views.py", line 305, in predicate_wrapper
    return view(context, request)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/pyramid/config/views.py", line 355, in rendered_view
    result = view(context, request)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/pyramid/config/views.py", line 501, in _requestonly_view
    response = view(request)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/cornice/service.py", line 527, in wrapper
    response = view_()
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/turnip/api/views.py", line 33, in validate_path_decorator
    return func(self, self.repo_store, name)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/turnip/api/views.py", line 239, in get
    sha1_prerequisite=sha1_prerequisite)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/turnip/api/store.py", line 446, in get_merge_diff
    conflicts = _add_conflicted_files(repo, merged_index)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/turnip/api/store.py", line 402, in _add_conflicted_files
    index.remove(path)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/pygit2/index.py", line 175, in remove
    check_error(err, True)
  File "/srv/turnip/turnip-api/payloads/d4c917cf38f9395dc6641c444a1850321a62a62d/env/local/lib/python2.7/site-packages/pygit2/errors.py", line 53, in check_error
    raise IOError(message)
IOError: index does not contain lib/lp/archivepublisher/archivesigningkey.py at stage 0

Further investigation revealed that I'd never added a specific test case for delete/delete conflicts (perhaps I had a hard time working out how to provoke one?), and so the code to handle them is very likely broken.