Improve tab-completion for DEALLOCATE.

Author: Naoki Nakamichi
Reviewed-by: Fujii Masao
Discussion: https://postgr.es/m/ec1a45b06edfce13706f2c765778d8c2@oss.nttdata.com

src/bin/psql/tab-complete.c

@@ -2911,7 +2911,8 @@ psql_completion(const char *text, int start, int end)
 
 /* DEALLOCATE */
 	else if (Matches("DEALLOCATE"))
-		COMPLETE_WITH_QUERY(Query_for_list_of_prepared_statements);
+		COMPLETE_WITH_QUERY(Query_for_list_of_prepared_statements
+							" UNION SELECT 'ALL'");
 
 /* DECLARE */
 	else if (Matches("DECLARE", MatchAny))