How to use the dulwich.objects.S_ISGITLINK function in dulwich

def content(mode, hexsha):
        if hexsha is None:
            return Blob.from_string(b'')
        elif S_ISGITLINK(mode):
            return Blob.from_string(b"Subproject commit " + hexsha + b"\n")
        else:
            return store[hexsha]

def pack_info_create(pack_data, pack_index):
    pack = Pack.from_objects(pack_data, pack_index)
    info = {}
    for obj in pack.iterobjects():
        # Commit
        if obj.type_num == Commit.type_num:
            info[obj.id] = (obj.type_num, obj.parents, obj.tree)
        # Tree
        elif obj.type_num == Tree.type_num:
            shas = [(s, n, not stat.S_ISDIR(m)) for
                    n, m, s in obj.items() if not S_ISGITLINK(m)]
            info[obj.id] = (obj.type_num, shas)
        # Blob
        elif obj.type_num == Blob.type_num:
            info[obj.id] = None
        # Tag
        elif obj.type_num == Tag.type_num:
            info[obj.id] = (obj.type_num, obj.object[1])
    return zlib.compress(json_dumps(info))

def content(mode, hexsha):
        if hexsha is None:
            return Blob.from_string(b'')
        elif S_ISGITLINK(mode):
            return Blob.from_string(b"Subproject commit " + hexsha + b"\n")
        else:
            return store[hexsha]

def _find_content_rename_candidates(self):
        candidates = self._candidates = []
        # TODO: Optimizations:
        #  - Compare object sizes before counting blocks.
        #  - Skip if delete's S_IFMT differs from all adds.
        #  - Skip if adds or deletes is empty.
        # Match C git's behavior of not attempting to find content renames if
        # the matrix size exceeds the threshold.
        if not self._should_find_content_renames():
            return

        block_cache = {}
        check_paths = self._rename_threshold is not None
        for delete in self._deletes:
            if S_ISGITLINK(delete.old.mode):
                continue  # Git links don't exist in this repo.
            old_sha = delete.old.sha
            old_obj = self._store[old_sha]
            block_cache[old_sha] = _count_blocks(old_obj)
            for add in self._adds:
                if stat.S_IFMT(delete.old.mode) != stat.S_IFMT(add.new.mode):
                    continue
                new_obj = self._store[add.new.sha]
                score = _similarity_score(old_obj, new_obj,
                                          block_cache=block_cache)
                if score > self._rename_threshold:
                    new_type = self._rename_type(check_paths, delete, add)
                    rename = TreeChange(new_type, delete.old, add.new)
                    candidates.append((-score, rename))

def get_nodes(self, path):
        if self._get_kind(path) != NodeKind.DIR:
            raise ChangesetError("Directory does not exist for revision %s at "
                " '%s'" % (self.revision, path))
        path = self._fix_path(path)
        id = self._get_id_for_path(path)
        tree = self.repository._repo[id]
        dirnodes = []
        filenodes = []
        als = self.repository.alias
        for name, stat, id in tree.iteritems():
            if objects.S_ISGITLINK(stat):
                dirnodes.append(SubModuleNode(name, url=None, changeset=id,
                                              alias=als))
                continue

            obj = self.repository._repo.get_object(id)
            if path != '':
                obj_path = '/'.join((path, name))
            else:
                obj_path = name
            if obj_path not in self._stat_modes:
                self._stat_modes[obj_path] = stat
            if isinstance(obj, objects.Tree):
                dirnodes.append(DirNode(obj_path, changeset=self))
            elif isinstance(obj, objects.Blob):
                filenodes.append(FileNode(obj_path, changeset=self, mode=stat))
            else:

def next(self):
        while True:
            if not self.objects_to_send:
                return None
            (sha, name, leaf) = self.objects_to_send.pop()
            if sha not in self.sha_done:
                break
        if not leaf:
            o = self.object_store[sha]
            if isinstance(o, Commit):
                self.add_todo([(o.tree, "", False)])
            elif isinstance(o, Tree):
                self.add_todo([(s, n, not stat.S_ISDIR(m))
                               for n, m, s in o.iteritems()
                               if not S_ISGITLINK(m)])
            elif isinstance(o, Tag):
                self.add_todo([(o.object[1], None, False)])
        if sha in self._tagged:
            self.add_todo([(self._tagged[sha], None, True)])
        self.sha_done.add(sha)
        self.progress(("counting objects: %d\r" %
                       len(self.sha_done)).encode('ascii'))
        return (sha, name)