[quodlibetor]

If you start doing a commit (via `c` in the magit status buffer, with the standard semantics of "you're going to commit everything that's currently staged") you can press capital F for an instant fixup, or capital S for instant squash.

When you press either of those, magit pops up a commit picker which shows the current git log. Selecting a commit will then instantaneously apply your staged changes to the selected commit. It's much simpler than any of the other workflows I've seen in response to your question.

The gif in this repo (for a tool I made that simulates this behavior as a cli tool for some jealous coworkers) tries to show the workflow: https://github.com/quodlibetor/git-fixup

That said, this _doesn't_ support the "automatically figure out which commits to apply hunks to" workflow. I personally find that I use both workflows depending on the nature of my changes.