Delitifier broken (Re: diff-core segfault)

[Junio C Hamano <junkio@cox.net>]

Junio C Hamano <junkio@cox.net> writes:

> Darrin Thompson <darrint@progeny.com> writes:
>
>> Could someone confirm that this exists on more recent git heads and fix
>> if needed?
>
> (1) Yup.  I can reproduce it.
> (2) Will look into it when able.

This is not just "diff".  Our deltify code is half-broken, and
in the worst case this can corrupt our packs if an empty blob is
involved.

The problem is if from_size or to_size is empty, it does not
produce any.

        if (!from_size || !to_size || delta_prepare(from_buf, from_size, &bdf))
                return NULL;
	

I think either we need to make the users more careful or fix
deltifier to produce trivial delta.  I'd vote for the latter;
let me rig up something.