Bug#28893 --relay-log variable is not exposed with SHOW VARIABLES

added variables relay_log, relay_log_index, relay_log_info_file to init_vars[]
to make them visible within SHOW VARIABLES

mysql-test/r/rpl_flush_log_loop.result

@@ -4,6 +4,13 @@ reset master;
 reset slave;
 drop table if exists t1,t2,t3,t4,t5,t6,t7,t8,t9;
 start slave;
+show variables like 'relay_log%';
+Variable_name	Value
+relay_log	MYSQLTEST_VARDIR/master-data/relay-log
+relay_log_index	
+relay_log_info_file	relay-log.info
+relay_log_purge	ON
+relay_log_space_limit	0
 stop slave;
 change master to master_host='127.0.0.1',master_user='root',
 master_password='',master_port=MASTER_PORT;

mysql-test/t/rpl_flush_log_loop.test

@@ -3,6 +3,9 @@
 
 source include/master-slave.inc;
 
+--replace_result $MYSQLTEST_VARDIR MYSQLTEST_VARDIR
+show variables like 'relay_log%';
+
 connection slave;
 stop slave;
 --replace_result $MASTER_MYPORT MASTER_PORT

sql/set_var.cc

@@ -1040,6 +1040,9 @@ struct show_var_st init_vars[]= {
   {sys_readonly.name,         (char*) &sys_readonly,                SHOW_SYS},
   {sys_read_rnd_buff_size.name,(char*) &sys_read_rnd_buff_size,	    SHOW_SYS},
 #ifdef HAVE_REPLICATION
+  {"relay_log" , (char*) &opt_relay_logname, SHOW_CHAR_PTR},
+  {"relay_log_index", (char*) &opt_relaylog_index_name, SHOW_CHAR_PTR},
+  {"relay_log_info_file", (char*) &relay_log_info_file, SHOW_CHAR_PTR},
   {sys_relay_log_purge.name,  (char*) &sys_relay_log_purge,         SHOW_SYS},
   {"relay_log_space_limit",  (char*) &relay_log_space_limit,        SHOW_LONGLONG},
 #endif