notes: refactor display notes default handling

notes: refactor display notes default handling

This is in preparation for more notes-related revision
command-line options.

The "suppress_default_notes" option is renamed to
"use_default_notes", and is now a tri-state with values less
than one indicating "not set".  If the value is "not set",
then we show default refs if and only if no other refs were
given.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>