With a clean magit-status ¹:

1. Move the point to the commit.
2. Revert changes with r.
3. Initiate the commit with c c.
4. Execute M-x magit-generate-changelog RET. Now C-c C-i followed by l g (git-commit-insert-changelog-gnu)
5. Copy the generate changelog.
6. Cancel the commit with C-c C-k.
7. Reset back to the clean magit-status.

I now recommend the new alternatives, , which use Magit’s own implementation and are accessible from the same transient menu used to insert commit trailers (`git-commit-insert-trailer` on `C-c C-i` (hm, maybe I should rename that now)).