psql: Add tab completion for COPY (MERGE ...

The underlying feature for this was added in PostgreSQL 17.

Author: Jian He <jian.universality@gmail.com>
Discussion: https://www.postgresql.org/message-id/CACJufxEmNjxvf1deR1zBrJbjAMeCooooLRzZ+yaaBuqDKh_6-Q@mail.gmail.com

Author: petere

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

@@ -3204,7 +3204,7 @@ match_previous_words(int pattern_id,
 		COMPLETE_WITH_SCHEMA_QUERY_PLUS(Query_for_list_of_tables, "(");
 	/* Complete COPY ( with legal query commands */
 	else if (Matches("COPY|\\copy", "("))
-		COMPLETE_WITH("SELECT", "TABLE", "VALUES", "INSERT INTO", "UPDATE", "DELETE FROM", "WITH");
+		COMPLETE_WITH("SELECT", "TABLE", "VALUES", "INSERT INTO", "UPDATE", "DELETE FROM", "MERGE INTO", "WITH");
 	/* Complete COPY <sth> */
 	else if (Matches("COPY|\\copy", MatchAny))
 		COMPLETE_WITH("FROM", "TO");