notes: Don't create (empty) commit when removing non-existing notes

notes: Don't create (empty) commit when removing non-existing notes

Extend remove_note() in the notes API to return whether or not a note was
actually removed. Use this in 'git notes remove' to skip the creation of
a notes commit when no notes were actually removed.

Also add a test illustrating the change in behavior.

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>