This option tells xtrabackup to create an incremental backup, rather
than a full one. It is passed to the xtrabackup child process. When
this option is specified, either --incremental-lsn or
--incremental-basedir can also be given. If neither option is given,
option --incremental-basedir is passed to xtrabackup by default, set
to the first timestamped backup directory in the backup base
directory.