packfile: always declare object info to be OI_PACKED

When reading object info via a packfile we yield one of two types:

  - The object can either be OI_PACKED, which is what a caller would
    typically expect.

  - Or it can be OI_DBCACHED if it is stored in the delta base cache.

The latter really is an implementation detail though, and callers
typically don't care at all about the difference. Furthermore, the
information whether or not it is part of the delta base cache can
already be derived via the `is_delta` field, so the fact that we discern
between OI_PACKED and OI_DBCACHED only further complicates the
interface.

There aren't all that many callers that care about the `whence` field in
the first place. In fact, there's only three:

  - `packfile_store_read_object_info()` checks for `whence == OI_PACKED`
    and then populates the packfile information of the object info
    structure. We now start to do this also for deltified objects, which
    gives its callers strictly more information.

  - `repack_local_links()` wants to determine whether the object is part
    of a promisor pack and checks for `whence == OI_PACKED`. If so, it
    verifies that the packfile is a promisor pack. It's arguably wrong
    to declare that an object is not part of a promisor pack only
    because it is stored in the delta base cache.

  - `is_not_in_promisor_pack_obj()` does the same, but checks that a
    specific object is _not_ part of a promisor pack. The same reasoning
    as above applies.

Drop the OI_DBCACHED enum completely. None of the callers seem to care
about the distinction.

Note that this also fixes a segfault introduced in 8c1b84bc97
(streaming: move logic to read packed objects streams into backend,
2025-11-23), which refactors how we stream packed objects. The intent is
to only read packed objects in case they are stored non-deltified as
we'd otherwise have to deflate them first. But the check for whether or
not the object is stored as a delta was unconditionally done via
`oi.u.packed.is_delta`, which is only valid in case `oi.whence` is
`OI_PACKED`. But under some circumstances we got `OI_DBCACHED` here,
which means that none of the `oi.u.packed` fields were initialized at
all. Consequently, we assumed the object was not stored as a delta, and
then try to read the object from `oi.u.packed.pack`, which is a `NULL`
pointer and thus causes a segfault.

Add a test case for this issue so that this cannot regress in the
future anymore.

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

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