add: use advise_if_enabled for ADVICE_ADD_EMBEDDED_REPO

add: use advise_if_enabled for ADVICE_ADD_EMBEDDED_REPO

By following a similar reasoning as in previous commits, there are no
reason why we should not use the advise_if_enabled() API to display the
ADVICE_ADD_EMBEDDED_REPO advice.

This advice was introduced in 532139940c (add: warn when adding an
embedded repository, 2017-06-14).  Some tests were included in the
commit, but none is testing this advice.  Which, note, we only want to
display once per run.

So, use the advise_if_enabled() machinery to show the
ADVICE_ADD_EMBEDDED_REPO advice and include a test to notice any
possible breakage.

Signed-off-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Rubén Justo <rjusto@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>