Idea: "git commit fixup last" alias

It'd be useful to have an alias that finds the last commit and makes this next commit a fixup for it to be used with `git rebase --autosquash` automatic fixup squashing...

e.g. `gfl` for `git-fixup-last` or something, which:
* Does `git log` to extract most recent SHA on this branch and the files affected by it
* Does a `git commit` on those files with `--fixup=$sha` to mark it as a fixup