git-absorb seems the more popular and more actively maintained implementation of this very useful concept.

i use git-absorb from inside emacs all the time via magit. it comes built-in but to use it directly from the menu needs a bit of convincing to let the shortcut invoke the right tool:

(use-package magit
  :config
  (transient-replace-suffix 'magit-commit 'magit-commit-autofixup
    '(6 "x" "Absorb changes" magit-commit-absorb)))

Interesting, I use fixup quite a lot and this may save some manual work