builtin/am: allow disabling conflict advice

builtin/am: allow disabling conflict advice

When 'git am' or 'git rebase --apply' encounter a conflict, they show a
message instructing the user how to continue the operation. This message
can't be disabled.

Use ADVICE_MERGE_CONFLICT introduced in the previous commit to allow
disabling it. Update the tests accordingly, as the advice output is now
on stderr instead of stdout. In t4150, redirect stdout to 'out' and
stderr to 'err', since this is less confusing. In t4254, as we are
testing a specific failure mode of 'git am', simply disable the advice.
Note that we are not testing that this advice is shown in 'git rebase'
for the apply backend since 2ac0d6273f (rebase: change the default
backend from "am" to "merge", 2020-02-15).

Helped-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Helped-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>