notes-cache: convert to struct object_id

notes-cache: convert to struct object_id

Convert as many instances of unsigned char [20] as possible.  Update the
callers of notes_cache_get and notes_cache_put to use the new interface.
Among the functions updated are callers of
lookup_commit_reference_gently, which we will soon convert.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>