lib-index: "first saved UID of the day" wasn't updated right always.

It was using the UID of the first append. But it may not have been the
lowest UID. Do this after the appends are first sorted by their UID.

src/lib-index/mail-index-transaction-finish.c

@@ -326,8 +326,8 @@ mail_index_transaction_convert_to_uids(struct mail_index_transaction *t)
 void mail_index_transaction_finish(struct mail_index_transaction *t)
 {
 	if (array_is_created(&t->appends)) {
-		mail_index_update_day_headers(t);
 		mail_index_transaction_sort_appends(t);
+		mail_index_update_day_headers(t);
 	}
 	mail_index_transaction_finish_flag_updates(t);