builtin-notes: Add "add" subcommand for adding notes to objects

builtin-notes: Add "add" subcommand for adding notes to objects

"git notes add" is identical to "git notes edit" except that instead of
editing existing notes for a given object, you can only add notes to an
object that currently has none. If "git notes add" finds existing notes
for the given object, the addition is aborted. However, if the new
-f/--force option is used, "git notes add" will _overwrite_ the existing
notes with the new notes contents.

If there is no existing notes for the given object. "git notes add" is
identical to "git notes edit" (i.e. it adds a new note).

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

Suggested-by: Joey Hess <joey@kitenet.net>
Improved-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>