Create a fixup commit and autosquash it into the target commit via rebase.
#!/usr/bin/env sh
# Usage: git-fixup <fixup_commit> [squash_commit]
fixup_commit="${1:-HEAD~1}"
squash_commit="${2:-HEAD~3}"
git add .
git commit --fixup "$fixup_commit"
git rebase -i "$squash_commit" --autosquash