notes: convert internal parts to struct object_id

notes: convert internal parts to struct object_id

Convert several portions of the internals of the code to struct
object_id.  Introduce two macros to denote the different constants in
the code: KEY_INDEX for the last byte of the object ID, and
FANOUT_PATH_SEPARATORS for the number of possible path separators (on
Unix, "/").  While these constants are both 19 (one less than the number
of bytes in the hash), distinguish them to make the code more
understandable, and define them logically based on their intended
purpose.

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