<SYSTEM_TASK:>
Solve the following problem using Python, implementing the functions described below, one line at a time
<END_TASK>
<USER_TASK:>
Description:

def dir_list(self, tgt_env):
        '''
        Get a list of directories for the target environment using pygit2
        '''
        def _traverse(tree, blobs, prefix):
            '''
            Traverse through a pygit2 Tree object recursively, accumulating all
            the empty directories within it in the "blobs" list
            '''
            for entry in iter(tree):
                if entry.oid not in self.repo:
                    # Entry is a submodule, skip it
                    continue
                blob = self.repo[entry.oid]
                if not isinstance(blob, pygit2.Tree):
                    continue
                blobs.append(
                    salt.utils.path.join(prefix, entry.name, use_posixpath=True)
                )
                if blob:
                    _traverse(
                        blob, blobs, salt.utils.path.join(
                            prefix, entry.name, use_posixpath=True)
                    )

        ret = set()
        tree = self.get_tree(tgt_env)
        if not tree:
            return ret
        if self.root(tgt_env):
            try:
                oid = tree[self.root(tgt_env)].oid
                tree = self.repo[oid]
            except KeyError:
                return ret
            if not isinstance(tree, pygit2.Tree):
                return ret
            relpath = lambda path: os.path.relpath(path, self.root(tgt_env))
        else:
            relpath = lambda path: path
        blobs = []
        if tree:
            _traverse(tree, blobs, self.root(tgt_env))
        add_mountpoint = lambda path: salt.utils.path.join(
            self.mountpoint(tgt_env), path, use_posixpath=True)
        for blob in blobs:
            ret.add(add_mountpoint(relpath(blob)))
        if self.mountpoint(tgt_env):
            ret.add(self.mountpoint(tgt_env))
        return ret