packfile: extend `is_delta` field to allow for "unknown" state

The `struct object_info::u::packed::is_delta` field determines whether or not a specific object is stored as a delta. It only stores whether or not the object is stored as delta, so it is treated as a boolean value. This boolean is insufficient though: when reading a packed object via `packfile_store_read_object_info()` we know to skip parsing the actual object when the user didn't request any object-specific data. In that case we won't read the object itself, but will only look up its position in the packfile. Consequently, we do not know whether it is a delta or not. This isn't really an issue right now, as the check for an empty request is broken. But a subsequent commit will fix it, and once we do we will have the need to also represent an "unknown" delta state. Prepare for this change by introducing a new enum that encodes the object type. We don't use the "unknown" state just yet, but will start to do so in a subsequent commit. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>