builtin-notes: Add -c/-C options for reusing notes

builtin-notes: Add -c/-C options for reusing notes

Inspired by the -c/-C options to "git commit", we teach these options to
"git notes add/append" to allow reuse of note objects.

With this patch in place, it is now easy to copy or move notes between
objects. For example, to copy object A's notes to object B:
	git notes add [-f] -C $(git notes list A) B
To move instead of copying, you simply remove the notes from the source
object afterwards, e.g.:
	git notes remove A

The patch includes tests verifying correct behaviour of the new functionality.

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