revision.c: support --notes command-line option

revision.c: support --notes command-line option

We already have --show-notes, but it has a few shortcomings:

  1. Using --show-notes=<ref> implies that we should also
     show the default notes. Which means you also need to
     use --no-standard-notes if you want to suppress them.

  2. It is negated by --no-notes, which doesn't match.

  3. It's too long to type. :)

This patch introduces --notes, which behaves exactly like
--show-notes, except that using "--notes=<ref>" does not
imply showing the default notes.

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