def down(context, repo_url):
    """(download) Synchronise remote repo to local repo.

    If repo_url is given, then clone from remote URL.
    """
    if repo_url == '':
        context.obj.find_repo_type()
        if context.obj.vc_name == 'git':
            context.obj.call(['git', 'pull'])
        elif context.obj.vc_name == 'hg':
            context.obj.call(['hg', 'pull', '-u'])
    else:
        context.obj.call([context.obj.vc_name, 'clone', repo_url])