For file contents, return the comment system.

def _comment_system_for_file(contents):
    """For file contents, return the comment system."""
    if contents[0] == "#":
        return FileCommentSystem(begin="#", middle="", end="", single="#")
    elif contents[:2] == "/*":
        return FileCommentSystem(begin="/*", middle="*", end="*/", single="//")
    elif contents[:2] == "//":
        return FileCommentSystem(begin="//", middle="//", end="", single="//")
    elif contents[:3] == "rem":
        return FileCommentSystem(begin="rem",
                                 middle="rem",
                                 end="",
                                 single="rem")
    else:
        raise RuntimeError("Couldn't detect comment "
                           "system from {0}".format(contents[:3]))