psql: Add completion support for AT [ LOCAL | TIME ZONE ]

AT TIME ZONE is completed with a list of supported timezones, something
not needed by AT LOCAL.

Author: Dagfinn Ilmari Mannsåker
Reviewed-by: Jim Jones
Discussion: https://postgr.es/m/87jzyzsvgv.fsf@wibble.ilmari.org

src/bin/psql/tab-complete.c

@@ -4682,6 +4682,12 @@ psql_completion(const char *text, int start, int end)
 	else if (TailMatches("JOIN"))
 		COMPLETE_WITH_SCHEMA_QUERY(Query_for_list_of_selectables);
 
+/* ... AT [ LOCAL | TIME ZONE ] ... */
+	else if (TailMatches("AT"))
+		COMPLETE_WITH("LOCAL", "TIME ZONE");
+	else if (TailMatches("AT", "TIME", "ZONE"))
+		COMPLETE_WITH_TIMEZONE_NAME();
+
 /* Backslash commands */
 /* TODO:  \dc \dd \dl */
 	else if (TailMatchesCS("\\?"))