environment: stop storing "core.notesRef" globally

environment: stop storing "core.notesRef" globally

Stop storing the "core.notesRef" config value globally. Instead,
retrieve the value in `default_notes_ref()`. The code is never called in
a hot loop anyway, so doing this on every invocation should be perfectly
fine.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>