Re: Add "--show-all" revision walker flag for debugging

[Junio C Hamano <gitster@pobox.com>]

Linus Torvalds <torvalds@linux-foundation.org> writes:

> The real issue is that it doesn't work. I thought about it, and with 
> multiple roots (which _can_ get merged together) it just isn't something 
> that actually helps.
>
> If you couldn't merge across roots, you could have a "uuid+generation 
> header", but the moment you have multiple roots it actually gets quite 
> complex.
>
> So scratch the generation header.  It's not the answer.

I do not think multiple roots can be helped without going all
the way down to the roots, and I think it can be proven.

> And I do think that we can do it without it. I'm still thinking about how 
> to do it efficiently, but I think I can get there. I just haven't had the 
> time to sit down and really think it through and try out my ideas yet.

Generation number would on the other hand eliminate the issue of
clock skew if heads are connected, and I think it also can be
proven.

Both proofs need the axiom "there is no loop in the commit
ancestry graph", but that is something we already rely on in the
merge-base computation.