omit system invisible fields from the duplicate key error

just as SHOW CREATE TABLE omits them from the index definition

mysql-test/suite/versioning/r/load_data.result

@@ -3,6 +3,6 @@ INSERT IGNORE INTO t1 (a,b,c) VALUES (1,2,3);
 SELECT a, b, c FROM t1 INTO OUTFILE '15330.data';
 LOAD DATA INFILE '15330.data' IGNORE INTO TABLE t1 (a,b,c);
 Warnings:
-Warning	1062	Duplicate entry '1-2038-01-19 04:14:07.999999' for key 'a'
+Warning	1062	Duplicate entry '1' for key 'a'
 LOAD DATA INFILE '15330.data' REPLACE INTO TABLE t1 (a,b,c);
 DROP TABLE t1;

sql/key.cc

@@ -427,6 +427,8 @@ void key_unpack(String *to, TABLE *table, KEY *key)
        key_part < key_part_end;
        key_part++)
   {
+    if (key_part->field->invisible > INVISIBLE_USER)
+      continue;
     if (to->length())
       to->append('-');
     if (key_part->null_bit)