fsck: accept an oid instead of a "struct blob" for fsck_blob()

fsck: accept an oid instead of a "struct blob" for fsck_blob()

We don't actually need any information from the object struct except its
oid (and the type, of course, but that's implicitly OBJ_BLOB). This
gives our callers more flexibility to drop the object structs, too.

Signed-off-by: Jeff King <redacted>
Signed-off-by: Junio C Hamano <redacted>