I mentioned elsewhere, but I made a simple bash function that does something like that but matches a string in a previous commit message (because I found that easier to type quickly than a commit hash):

  function git-commit-fixup() {
    git commit --fixup ":/$*"
  }
  # usage: suppose there's a commit "fix: the thing"
  git-commit-fixup thing
  # now there's a new commit "fixup! fix: the thing" which can be autosquashed