[PATCH]Feature improvement for MERGE tab completion

Hi!

I created a patch for improving MARGE tab completion.
Currently there is a problem with "MERGE INTO dst as d Using src as s ON 
d.key = s.key WHEN <tab>" is typed, "MATCHED" and "NOT MATCHED" is not 
completed.
There is also a problem that typing "MERGE INTO a AS <tab>" completes 
"USING".
This patch solves the above problems.

Regards,
Kotaro Kawamoto