notes: dry-run and verbose options for prune

notes: dry-run and verbose options for prune

Introduce -n and -v options for "git notes prune" in complete analogy to
"git prune" so that one can check for dangling notes easily.

The output is a list of names of objects whose notes would be resp.
are removed so that one can check the object ("git show sha1") as well as
the note ("git notes show sha1").

Signed-off-by: Michael J Gruber <git@drmicha.warpmail.net>
Acked-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>