For blobs, we want to make sure the on-disk data is not corrupted
(i.e. can be inflated and produce the expected SHA-1). Blob content is
opaque, there's nothing else inside to check for.

For really large blobs, we may want to avoid unpacking the entire blob
in memory, just to check whether it produces the same SHA-1. On 32-bit
systems, we may not have enough virtual address space for such memory
allocation. And even on 64-bit where it's not a problem, allocating a
lot more memory could result in kicking other parts of systems to swap
file, generating lots of I/O and slowing everything down.

For this particular operation, not unpacking the blob and letting
check_sha1_signature, which supports streaming interface, do the job
is sufficient. check_sha1_signature() is not shown in the diff,
unfortunately. But if will be called when "data_valid && !data" is
false.

We will call the callback function "fn" with NULL as "data". The only
callback of this function is fsck_obj_buffer(), which does not touch
"data" at all if it's a blob.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>