builtin-notes: Add "append" subcommand for appending to note objects

builtin-notes: Add "append" subcommand for appending to note objects

"git notes append" is equivalent to "git notes edit" except that instead
of editing existing notes contents, you can only append to it. This is
useful for quickly adding annotations like e.g.:
	git notes append -m "Acked-by: A U Thor <author@example.com>"

"git notes append" takes the same -m/-F options as "git notes add".

If there is no existing note to append to, "git notes append" is identical
to "git notes add" (i.e. it adds a new note).

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>