psql: Improve CREATE INDEX CONCURRENTLY tab completion

The completion of CREATE INDEX CONCURRENTLY was lacking in several ways
compared to a plain CREATE INDEX command:

- CREATE INDEX <name> ON completes table names, but didn't with
  CONCURRENTLY.

- CREATE INDEX completes ON and existing index names, but with
  CONCURRENTLY it only completed ON.

- CREATE INDEX <name> completes ON, but didn't with CONCURRENTLY.

These are now all fixed.

src/bin/psql/tab-complete.c

@@ -2005,15 +2005,17 @@ psql_completion(const char *text, int start, int end)
 		COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_indexes,
 								   " UNION SELECT 'ON'"
 								   " UNION SELECT 'CONCURRENTLY'");
-	/* Complete ... INDEX [<name>] ON with a list of tables  */
+	/* Complete ... INDEX|CONCURRENTLY [<name>] ON with a list of tables  */
-	else if (TailMatches3("INDEX", MatchAny, "ON") ||
+	else if (TailMatches3("INDEX|CONCURRENTLY", MatchAny, "ON") ||
 			 TailMatches2("INDEX|CONCURRENTLY", "ON"))
 		COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_tm, NULL);
-	/* If we have CREATE|UNIQUE INDEX CONCURRENTLY, then add "ON" */
+	/* Complete ... INDEX CONCURRENTLY with "ON" and existing indexes */
 	else if (TailMatches2("INDEX", "CONCURRENTLY"))
-		COMPLETE_WITH_CONST("ON");
+		COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_indexes,
-	/* If we have CREATE|UNIQUE INDEX <sth>, then add "ON" */
+								   " UNION SELECT 'ON'");
-	else if (TailMatches3("CREATE|UNIQUE", "INDEX", MatchAny))
+	/* Complete CREATE|UNIQUE INDEX [CONCURRENTLY] <sth> with "ON" */
+	else if (TailMatches3("CREATE|UNIQUE", "INDEX", MatchAny) ||
+			 TailMatches4("CREATE|UNIQUE", "INDEX", "CONCURRENTLY", MatchAny))
 		COMPLETE_WITH_CONST("ON");
 
 	/*