MDEV-9168 altering a column comment does a full copy

add a test case

mysql-test/r/alter_table_online.result

@@ -64,3 +64,12 @@ drop table t1,t2,t3;
 create table t1 (i int) partition by hash(i) partitions 2;
 alter online table t1 comment 'test';
 drop table t1;
+create table t1 (a int);
+alter online table t1 modify a int comment 'test';
+drop table t1;
+create table t1 (a int) engine=innodb;
+alter online table t1 modify a int comment 'test';
+drop table t1;
+create table t1 (a int) partition by hash(a) partitions 2;
+alter online table t1 modify a int comment 'test';
+drop table t1;

mysql-test/t/alter_table_online.test

@@ -106,3 +106,18 @@ drop table t1,t2,t3;
 create table t1 (i int) partition by hash(i) partitions 2;
 alter online table t1 comment 'test';
 drop table t1;
+
+#
+# MDEV-9168 altering a column comment does a full copy
+#
+create table t1 (a int);
+alter online table t1 modify a int comment 'test';
+drop table t1;
+
+create table t1 (a int) engine=innodb;
+alter online table t1 modify a int comment 'test';
+drop table t1;
+
+create table t1 (a int) partition by hash(a) partitions 2;
+alter online table t1 modify a int comment 'test';
+drop table t1;