Return an iterator of policy changes along a commit lineage in a repo.

def delta_stream(self, target='HEAD', limit=None,
                     sort=pygit2.GIT_SORT_TIME | pygit2.GIT_SORT_REVERSE,
                     after=None, before=None):
        """Return an iterator of policy changes along a commit lineage in a repo.
        """
        if target == 'HEAD':
            target = self.repo.head.target

        commits = []
        for commit in self.repo.walk(target, sort):
            cdate = commit_date(commit)
            log.debug(
                "processing commit id:%s date:%s parents:%d msg:%s",
                str(commit.id)[:6], cdate.isoformat(),
                len(commit.parents), commit.message)
            if after and cdate > after:
                continue
            if before and cdate < before:
                continue
            commits.append(commit)
            if limit and len(commits) > limit:
                break

        if limit:
            self.initialize_tree(commits[limit].tree)
            commits.pop(-1)

        for commit in commits:
            for policy_change in self._process_stream_commit(commit):
                yield policy_change