By that definition of "derived information", the hash is "derived information" since it's based of the changes made to source data (whatever that data may be).
That said, point taken about the necessity of both generation numbers and timestamps. But that invalidates the OPs comment about git storing "only the minimum amount of information". It sounds like that's never been a hard principle.
Thanks for the background! (Seriously, not trying to be snarky.)
That info does support my original point that generational numbers probably should have been stored from the start and timestamps are the more "derivative" bit of information since it comes from the environment and not the data itself.
Thus, rlpb's concern that storing generational numbers pollutes its design of storing "only the minimum amount of information" isn't necessarily well founded, since the generational number might be more minimal and correct than the current timestamp. That was the aim of my original post: generational info is fundamental, not extraneous derived info, and probably have been stored with commits in the first place.
That said, point taken about the necessity of both generation numbers and timestamps. But that invalidates the OPs comment about git storing "only the minimum amount of information". It sounds like that's never been a hard principle.