I don't consider `commit --fixup` to be some arcane workaround, that is basically the default to record a change to some older commit.
Editing commits is also a normal, sane part of my workflow, what else is a version control system supposed to do? I consider modifying every commit in a repo not to be that frequent, but nice if JJ supports that easily. Do you want to educate us of the command?
Git also does certain modifications entirely in memory, but when I edit some file obviously my editor needs to access it. Also I want to rerun the tests on some modified commit anyway, so to me checking it out is not some extra cost.
