Bug#7943: Wrong prefix lengths reported on UTF-8 columns

SHOW KEYS FROM t1 now displays number of characters in Sub_part,
not number of bytes, to be compatible with SHOW CREATE TABLE.

mysql-test/r/ctype_mb.result

@@ -33,7 +33,7 @@ t1	CREATE TABLE `t1` (
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 SHOW KEYS FROM t1;
 Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment
-t1	1	key_a	1	a	A	NULL	9	NULL	YES	BTREE	
+t1	1	key_a	1	a	A	NULL	3	NULL	YES	BTREE	
 ALTER TABLE t1 CHANGE a a CHAR(4);
 SHOW CREATE TABLE t1;
 Table	Create Table
@@ -53,5 +53,5 @@ t1	CREATE TABLE `t1` (
 ) ENGINE=MyISAM DEFAULT CHARSET=latin1
 SHOW KEYS FROM t1;
 Table	Non_unique	Key_name	Seq_in_index	Column_name	Collation	Cardinality	Sub_part	Packed	Null	Index_type	Comment
-t1	1	key_a	1	a	A	NULL	9	NULL	YES	BTREE	
+t1	1	key_a	1	a	A	NULL	3	NULL	YES	BTREE	
 DROP TABLE t1;

sql/sql_show.cc

@@ -1025,7 +1025,8 @@ mysqld_show_keys(THD *thd, TABLE_LIST *table_list)
       /* Check if we have a key part that only uses part of the field */
       if (!(key_info->flags & HA_FULLTEXT) && (!key_part->field ||
           key_part->length != table->field[key_part->fieldnr-1]->key_length()))
-        protocol->store_tiny((longlong) key_part->length);
+        protocol->store_tiny((longlong) key_part->length / 
+                             key_part->field->charset()->mbmaxlen);
       else
         protocol->store_null();
       protocol->store_null();                   // No pack_information yet