advice: move advice.graftFileDeprecated squashing to commit.[ch]

advice: move advice.graftFileDeprecated squashing to commit.[ch]

Move the squashing of the advice.graftFileDeprecated advice over to an
external variable in commit.[ch], allowing advice() to purely use the
new-style API of invoking advice() with an enum.

See 8821e90a09a (advice: don't pointlessly suggest
--convert-graft-file, 2018-11-27) for why quieting this advice was
needed. It's more straightforward to move this code to commit.[ch] and
use it builtin/replace.c, than to go through the indirection of
advice.[ch].

Because this was the last advice_config variable we can remove that
old facility from advice.c.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>