pull: colorize the hint about setting `pull.rebase`

pull: colorize the hint about setting `pull.rebase`

In d18c950a69f (pull: warn if the user didn't say whether to rebase or
to merge, 2020-03-09), a new hint was introduced to encourage users to
make a conscious decision about whether they want their pull to merge or
to rebase by configuring the `pull.rebase` setting.

This warning was clearly intended to advise users, but as pointed out in
https://lore.kernel.org/git/87ima2rdsm.fsf%40evledraar.gmail.com, it
uses `warning()` instead of `advise()`.

One consequence is that the advice is not colorized in the same manner
as other, similar messages. So let's use `advise()` instead.

Pointed-out-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>