Comment 2 for bug 1411633

This is a consequence of the poor layout noted in this comment in purge_missing:

        if (will_run_mandb)
                /* Reset mtime to avoid confusing mandb into not running.
                 * TODO: It would be better to avoid this by only opening
                 * the database once between here and mandb.
                 */
                MYDBM_SET_TIME (dbf, db_mtime);

There are some situations where a later update_db will not actually modify the database and update the mtime, and in this situation purge_missing may have updated the database but reset the mtime to its previous value. The reorganisation implied by this comment would fix this.