fsck: provide a function to fsck buffer without object struct

fsck: provide a function to fsck buffer without object struct

The fsck code has been slowly moving away from requiring an object
struct in commits like 103fb6d43b (fsck: accept an oid instead of a
"struct tag" for fsck_tag(), 2019-10-18), c5b4269b57 (fsck: accept an
oid instead of a "struct commit" for fsck_commit(), 2019-10-18), etc.

However, the only external interface that fsck.c provides is
fsck_object(), which requires an object struct, then promptly discards
everything except its oid and type. Let's factor out the post-discard
part of that function as fsck_buffer(), leaving fsck_object() as a thin
wrapper around it. That will provide more flexibility for callers which
may not have a struct.

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