advice: allow disabling the automatic hint in advise_if_enabled()

advice: allow disabling the automatic hint in advise_if_enabled()

Using advise_if_enabled() to display an advice will automatically
include instructions on how to disable the advice, alongside the main
advice:

	hint: use --reapply-cherry-picks to include skipped commits
	hint: Disable this message with "git config advice.skippedCherryPicks false"

To do so, we provide a knob which can be used to disable the advice.

But also to tell us the opposite: to show the advice.

Let's not include the deactivation instructions for an advice if the
user explicitly sets its visibility.

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