"git notes prune" will remove all notes that annotate unreachable/non-
existing objects.
The patch includes tests verifying correct behaviour of the new subcommand.
Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>