Re: [PATCH v2 3/7] 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 the next commit.