Or you could record the original graph of commits during a rebase. When the actual content wasn't that important, you could choose to discard the old commit dat

Or you could record the original graph of commits during a rebase. When the actual content wasn't that important, you could choose to discard the old commit data but keep that metadata graph. (That way someone who builds on top of an old commit can use that graph to get the right merge logic.) But then you would have reimplemented mercurial's obsolescence markers.