Funny that I've been doing something nearly identical, but with way more boilerplate.

    fzfCommit() {
      local FZF_PROMPT="${FZF_PROMPT:=Commit: }"
      git log --oneline | fzf --border --prompt="$FZF_PROMPT" --height=10 --preview="git show {+1} --color=always" --no-sort --reverse | cut -d' ' -f1 | tr '\n' ' ' | sed 's/[[:space:]]$//';
    }
    function gfixup {
      local commit=$(FZF_PROMPT='Fixup Commit: ' fzfCommit)
      if [[ -z "$commit" ]]; then
        return 1
      fi
      set -x
      git commit --fixup "$commit" --allow-empty > /dev/null || return 1
      git rebase --interactive "$commit"~ --autosquash || return 1
    }