def _should_fetch(cls, dest, rev):
        """
        Return true if rev is a ref or is a commit that we don't have locally.

        Branches and tags are not considered in this method because they are
        assumed to be always available locally (which is a normal outcome of
        ``git clone`` and ``git fetch --tags``).
        """
        if rev.startswith("refs/"):
            # Always fetch remote refs.
            return True

        if not looks_like_hash(rev):
            # Git fetch would fail with abbreviated commits.
            return False

        if cls.has_commit(dest, rev):
            # Don't fetch if we have the commit locally.
            return False

        return True