Create a new revision file.

def revision(directory=None, message=None, autogenerate=False, sql=False,
             head='head', splice=False, branch_label=None, version_path=None,
             rev_id=None):
    """Create a new revision file."""
    config = current_app.extensions['migrate'].migrate.get_config(directory)
    if alembic_version >= (0, 7, 0):
        command.revision(config, message, autogenerate=autogenerate, sql=sql,
                         head=head, splice=splice, branch_label=branch_label,
                         version_path=version_path, rev_id=rev_id)
    else:
        command.revision(config, message, autogenerate=autogenerate, sql=sql)