The feature was backported from MySQL 5.6.
Some code was added to make commands as
        SELECT * FROM ignored_db.t1;
        CALL ignored_db.proc();
        USE ignored_db;
to take that option into account.

per-file comments:
  mysql-test/r/ignore_db_dirs_basic.result
        test result added.
  mysql-test/t/ignore_db_dirs_basic-master.opt
        options for the test,
        actually the set of --ignore-db-dir lines.
  mysql-test/t/ignore_db_dirs_basic.test
        test for the feature.
        Same test from 5.6 was taken as a basis,
        then tests for SELECT, CALL etc were added.

per-file comments:
  sql/mysql_priv.h
MDEV-495 backport --ignore-db-dir.
        interface for db_name_is_in_ignore_list() added.
  sql/mysqld.cc
MDEV-495 backport --ignore-db-dir.
        --ignore-db-dir handling.
  sql/set_var.cc
MDEV-495 backport --ignore-db-dir.
        the @@ignore_db_dirs variable added.
  sql/sql_show.cc
MDEV-495 backport --ignore-db-dir.
        check if the directory is ignored.
  sql/sql_show.h
MDEV-495 backport --ignore-db-dir.
        interface added for opt_ignored_db_dirs.
  sql/table.cc
MDEV-495 backport --ignore-db-dir.
        check if the directory is ignored.