Lightweight copy

[Guido Ostkamp]

> Overlay does some pretty similar work. I think it shouldn't be too hard 
> to adapt it for in-repo backpointers, say by setting another revlog flag 
> in entry 0 and making the inline data be the source name.

If you are going to implement this, it would be greatly appreciated. As 
already said in other posts, disk space is really expensive for me and 
many others.

If you need some volunteer for testing, I would happily do it.

Regards

Guido