psql: Improve tab-completion for PREPARE.

This commit enhances tab-completion for PREPARE xx AS to also suggest
MERGE INTO, VALUES, WITH, and TABLE.

Author: Haruna Miwa <miwa@sraoss.co.jp>
Reviewed-by: Fujii Masao <masao.fujii@gmail.com>
Discussion: https://postgr.es/m/TY7P286MB5466B859BD6C5BE64E961878F1CEA@TY7P286MB5466.JPNP286.PROD.OUTLOOK.COM

src/bin/psql/tab-complete.in.c

@@ -4951,7 +4951,8 @@ match_previous_words(int pattern_id,
 
 /* PREPARE xx AS */
 	else if (Matches("PREPARE", MatchAny, "AS"))
-		COMPLETE_WITH("SELECT", "UPDATE", "INSERT INTO", "DELETE FROM");
+		COMPLETE_WITH("SELECT", "UPDATE", "INSERT INTO", "DELETE FROM",
+					  "MERGE INTO", "VALUES", "WITH", "TABLE");
 
 /*
  * PREPARE TRANSACTION is missing on purpose. It's intended for transaction