advice: revamp advise API

advice: revamp advise API

Currently it's very easy for the advice library's callers to miss
checking the visibility step before printing an advice. Also, it makes
more sense for this step to be handled by the advice library.

Add a new advise_if_enabled function that checks the visibility of
advice messages before printing.

Add a new helper advise_enabled to check the visibility of the advice
if the caller needs to carry out complicated processing based on that
value.

A list of advice_settings is added to cache the config variables names
and values, it's intended to replace advice_config[] and the global
variables once we migrate all the callers to use the new APIs.

Signed-off-by: Heba Waly <heba.waly@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>