note 125267 deleted from features.gc.collecting-cycles by cmb

Note Submitter: Jack at example dot com ---- In step B, the algorithm runs a depth-first search on all possible roots to decrease by one the refcounts of each zval it finds, making sure not to decrease a refcount on the same zval twice (by marking them as "grey") 为了防止同一个zval紫色变量删除2次，每删除一次，就标记为灰色。