Extracts all the changes from your HEAD commit and unstages them, leaving the commit message in-place (albeit with a different sha, of course), so it's easier to go back and fix the commit with visual diff -enabled editors (since the changes will show up as unstaged, and will usually be colorized).

git-extract.sh

#!/usr/bin/env bash

git log --format=%B -n 1 HEAD > ._GIT_EXTRACT_MSG && git reset --soft HEAD^ && git reset HEAD && git commit --allow-empty -F ._GIT_EXTRACT_MSG && rm ._GIT_EXTRACT_MSG