notes: convert write_notes_tree to object_id

notes: convert write_notes_tree to object_id

Convert the definition and declaration of write_notes_tree to
struct object_id and adjust usage of this function.

Additionally, improve style of small part of this function, as old
formatting made it hard to understand at glance what this part of
code is doing.

Signed-off-by: Patryk Obara <patryk.obara@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>