cat-file: make --allow-unknown-type a noop

The cat-file command has some minor support for handling objects with
"unknown" types. I.e., strings that are not "blob", "commit", "tree", or
"tag".

In theory this could be used for debugging or experimenting with
extensions to Git. But in practice this support is not very useful:

  1. You can get the type and size of such objects, but nothing else.
     Not even the contents!

  2. Only loose objects are supported, since packfiles use numeric ids
     for the types, rather than strings.

  3. Likewise you cannot ever transfer objects between repositories,
     because they cannot be represented in the packfiles used for the
     on-the-wire protocol.

The support for these unknown types complicates the object-parsing code,
and has led to bugs such as b748ddb7a4 (unpack_loose_header(): fix
infinite loop on broken zlib input, 2025-02-25). So let's drop it.

The first step is to remove the user-facing parts, which are accessible
only via cat-file. This is technically backwards-incompatible, but given
the limitations listed above, these objects couldn't possibly be useful
in any workflow.

However, we can't just rip out the option entirely. That would hurt a
caller who ran:

  git cat-file -t --allow-unknown-object <oid>

and fed it normal, well-formed objects. There --allow-unknown-type was
doing nothing, but we wouldn't want to start bailing with an error. So
to protect any such callers, we'll retain --allow-unknown-type as a
noop.

The code change is fairly small (but we'll able to clean up more code in
follow-on patches). The test updates drop any use of the option. We
still retain tests that feed the broken objects to cat-file without
--allow-unknown-type, as we should continue to confirm that those
objects are rejected. Note that in one spot we can drop a layer of loop,
re-indenting the body; viewing the diff with "-w" helps there.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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