reversible binary diff

I just noticed that the binary diff format was augmented in order to 
carry the reverse diff information.

Why was this needed?

I mean, if you want to reverse a binary diff you only need to retrieve 
the original blob the forward diff was meant to apply against, and it is 
certainly already available in the object store if the forward diff has 
been previously applied.  Or has this assumption been wrong for some 
work flow?


Nicolas