def diff(repo_uri, source, target, output, verbose):
    """Policy diff between two arbitrary revisions.

    Revision specifiers for source and target can use fancy git refspec syntax
    for symbolics, dates, etc.

    See: https://git-scm.com/book/en/v2/Git-Tools-Revision-Selection

    Default revision selection is dependent on current working tree
    branch. The intent is for two use cases, if on a non-master branch
    then show the diff to master.  If on master show the diff to
    previous commit on master. For repositories not using the
    `master` convention, please specify explicit source and target.
    """
    logging.basicConfig(
        format="%(asctime)s: %(name)s:%(levelname)s %(message)s",
        level=(verbose and logging.DEBUG or logging.INFO))
    logging.getLogger('botocore').setLevel(logging.WARNING)

    if repo_uri is None:
        repo_uri = pygit2.discover_repository(os.getcwd())

    repo = pygit2.Repository(repo_uri)
    load_resources()

    # If on master show diff between last commit to current head
    if repo.head.shorthand == 'master':
        if source is None:
            source = 'master@{1}'
        if target is None:
            target = 'master'
    # Else show difference between master and current head
    elif target is None:
        target = repo.head.shorthand
    if source is None:
        source = 'master'

    policy_repo = PolicyRepo(repo_uri, repo)
    changes = list(policy_repo.delta_commits(
        repo.revparse_single(source), repo.revparse_single(target)))
    output.write(
        yaml.safe_dump({
            'policies': [c.policy.data for c in changes
                         if c.kind != ChangeType.REMOVE]}).encode('utf8'))