builtin/cat-file: support "blob:none" objects filter

builtin/cat-file: support "blob:none" objects filter

Implement support for the "blob:none" filter in git-cat-file(1), which
causes us to omit all blobs.

Note that this new filter requires us to read the object type via
`oid_object_info_extended()` in `batch_object_write()`. But as we try to
optimize away reading objects from the database the `data->info.typep`
pointer may not be set. We thus have to adapt the logic to conditionally
set the pointer in cases where the filter is given.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>