am: move advice.amWorkDir parsing back to advice.c

am: move advice.amWorkDir parsing back to advice.c

The only benefit from this move (apart from cleaner code) is that
advice.amWorkDir should now show up in `git help --config`. There
should be no regression since advice config is always read by the
git_default_config().

While at there, use advise() like other code. We now get "hint: "
prefix and the output is stderr instead of stdout (which is also the
reason for the test update because stderr is checked in a following
test and the extra advice can fail it).

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>