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>
diff --git a/advice.h b/advice.h
index 9f5064e..7e93778 100644
--- a/advice.h
+++ b/advice.h
@@ -17,6 +17,7 @@ extern int advice_implicit_identity;
 extern int advice_detached_head;
 extern int advice_set_upstream_failure;
 extern int advice_object_name_warning;
+extern int advice_amworkdir;
 extern int advice_rm_hints;
 extern int advice_add_embedded_repo;
 extern int advice_ignored_hook;