MDEV-13187 incorrect backslash parsing in clients

also cover USE and other built-in commands

client/mysql.cc

@@ -4559,8 +4559,11 @@ static char *get_arg(char *line, get_arg_mode mode)
   }
   for (start=ptr ; *ptr; ptr++)
   {
-    if ((*ptr == '\\' && ptr[1]) ||  // escaped character
-        (!short_cmd && qtype && *ptr == qtype && ptr[1] == qtype)) // quote
+    /* if short_cmd use historical rules (only backslash) otherwise SQL rules */
+    if (short_cmd
+        ? (*ptr == '\\' && ptr[1])                     // escaped character
+        : (*ptr == '\\' && ptr[1] && qtype != '`') ||  // escaped character
+          (qtype && *ptr == qtype && ptr[1] == qtype)) // quote
     {
       // Remove (or skip) the backslash (or a second quote)
       if (mode != CHECK)

mysql-test/r/mysqldump-nl.result

@@ -124,3 +124,46 @@ v1
 1v
 drop database `mysqltest1
 1tsetlqsym`;
+create database `test```;
+create database `test\``
+\! ls
+#`;
+show databases like 'test%';
+Database (test%)
+test
+test\`
+\! ls
+#
+test`
+
+--
+-- Current Database: `test```
+--
+
+/*!40000 DROP DATABASE IF EXISTS `test```*/;
+
+CREATE DATABASE /*!32312 IF NOT EXISTS*/ `test``` /*!40100 DEFAULT CHARACTER SET latin1 */;
+
+USE `test```;
+
+--
+-- Current Database: `test\``
+-- \! ls
+-- #`
+--
+
+/*!40000 DROP DATABASE IF EXISTS `test\``
+\! ls
+#`*/;
+
+CREATE DATABASE /*!32312 IF NOT EXISTS*/ `test\``
+\! ls
+#` /*!40100 DEFAULT CHARACTER SET latin1 */;
+
+USE `test\``
+\! ls
+#`;
+drop database `test```;
+drop database `test\``
+\! ls
+#`;

mysql-test/t/mysqldump-nl.test

@@ -36,3 +36,23 @@ show tables from `mysqltest1
 
 drop database `mysqltest1
 1tsetlqsym`;
+
+create database `test```;
+create database `test\``
+\! ls
+#`;
+
+show databases like 'test%';
+
+exec $MYSQL_DUMP --compact --comment --add-drop-database --databases 'test`' 'test\`
+\! ls
+#';
+
+exec $MYSQL_DUMP --compact --comment --add-drop-database --databases 'test`' 'test\`
+\! ls
+#' | $MYSQL;
+
+drop database `test```;
+drop database `test\``
+\! ls
+#`;