refs: don't store peeled object IDs for invalid tags

refs: don't store peeled object IDs for invalid tags

Both the "files" and "reftable" backend store peeled object IDs for
references that point to tags:

  - The "files" backend stores the value when packing refs, where each
    peeled object ID is prefixed with "^".

  - The "reftable" backend stores the value whenever writing a new
    reference that points to a tag via a special ref record type.

Both of these backends use `peel_object()` to find the peeled object ID.
But as explained in the preceding commit, that function does not detect
the case where the tag's tagged object and its claimed type mismatch.

The consequence of storing these bogus peeled object IDs is that we're
less likely to detect such corruption in other parts of Git.
git-for-each-ref(1) for example does not notice anymore that the tag is
broken when using "--format=%(*objectname)" to dereference tags.

One could claim that this is good, because it still allows us to mostly
use the tag as intended. But the biggest problem here is that we now
have different behaviour for such a broken tag depending on whether or
not we have its peeled value in the refdb.

Fix the issue by verifying the object type when peeling the object. If
that verification fails we simply skip storing the peeled value in
either of the reference formats.

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