Return SHA1 of filename.

def sha1(filename):
    """Return SHA1 of filename."""
    filename = unmap_file(filename)
    if filename not in file_cache:
        cache_file(filename)
        if filename not in file_cache:
            return None
        pass
    if file_cache[filename].sha1:
        return file_cache[filename].sha1.hexdigest()
    sha1 = hashlib.sha1()
    for line in file_cache[filename].lines['plain']:
        sha1.update(line.encode('utf-8'))
        pass
    file_cache[filename].sha1 = sha1
    return sha1.hexdigest()