Fix "log" family not to be too agressive about showing notes

Fix "log" family not to be too agressive about showing notes

Giving "Notes" information in the default output format of "log" and
"show" is a sensible progress (the user has asked for it by having the
notes), but for some commands (e.g. "format-patch") spewing notes into the
formatted commit log message without being asked is too aggressive.

Enable notes output only for "log", "show", "whatchanged" by default and
only when the user didn't ask any specific --pretty/--format from the
command line; users can explicitly override this default with --show-notes
and --no-notes option.

Parts of tests are taken from Jeff King's fix.

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