http-push: convert process_ls_object and descendants to object_id

http-push: convert process_ls_object and descendants to object_id

Rename one function to reflect that it now uses struct object_id.  This
conversion is a prerequisite for converting parse_object.

Note that while the use of a buffer that is exactly forty bytes long
looks questionable, get_oid_hex reads exactly the right number of bytes
and does not require the data to be NUL-terminated.

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