show grants should quote database name

[GMHDBJD]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

GRANT ALL PRIVILEGES ON `xx.xx`.* TO 'root'@'%';
SHOW GRANTS FOR 'root'@'%';

2. What did you expect to see? (Required)

GRANT ALL PRIVILEGES ON `xx.xx`.* TO 'root'@'%'

3. What did you see instead (Required)

GRANT ALL PRIVILEGES ON xx.xx.* TO 'root'@'%'

4. What is your TiDB version? (Required)

master

[djshow832]

This issue indeed affects more than one version, but these labels just make bug triage duplicated. I'll remove them.