Create an annotated tag at the current head.

def git_tag(repo_dir, tagname, message=None, force=True):
    """Create an annotated tag at the current head."""
    message = message or "%s" % tagname
    command = ['git', 'tag', '--annotate', '--message', message]
    if force:
        command.append('--force')
    # append the tag as the final arg
    command.append(tagname)
    return execute_git_command(command, repo_dir=repo_dir)