add: use advise_if_enabled for ADVICE_ADD_EMPTY_PATHSPEC
Since 93b0d86aaf (git-add: error out when given no arguments., 2006-12-20) we display a message when no arguments are given to "git add". Part of that message was converted to advice in bf66db37f1 (add: use advise function to display hints, 2020-01-07). Following the same line of reasoning as in the previous commit, it is sensible to use advise_if_enabled() here. Therefore, use advise_if_enabled() in builtin/add.c to show the ADVICE_ADD_EMPTY_PATHSPEC advice, and don't bother checking there the visibility of the advice or displaying the instruction on how to disable it. Also add a test for these messages, in order to detect a possible change in them. Signed-off-by: Rubén Justo <rjusto@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>