fsck: detect trailing garbage in all object types

fsck: detect trailing garbage in all object types

When a loose tree or commit is read by fsck (or any git
program), unpack_sha1_rest() checks whether there is extra
cruft at the end of the object file, after the zlib data.
Blobs that are streamed, however, do not have this check.

For normal git operations, it's not a big deal. We know the
sha1 and size checked out, so we have the object bytes we
wanted.  The trailing garbage doesn't affect what we're
trying to do.

But since the point of fsck is to find corruption or other
problems, it should be more thorough. This patch teaches its
loose-sha1 reader to detect extra bytes after the zlib
stream and complain.

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