ref-filter: use oid_object_info() to get object

ref-filter: use oid_object_info() to get object

Use oid_object_info_extended() to get object info instead of
read_object_file().
It will help to handle some requests faster (e.g., we do not need to
parse whole object if we need to know %(objectsize)).
It could also help us to add new atoms such as %(objectsize:disk)
and %(deltabase).

Signed-off-by: Olga Telezhnaia <olyatelezhnaya@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>