Git aliases to make new commits that fixup or are squashed into previous commits

.gitconfig

[alias]
  fixup = !sh -c 'REV=$(git rev-parse $1) && git commit --fixup $@ && git rebase -i --autosquash $REV^' -
  squash = !sh -c 'REV=$(git rev-parse $1) && git commit --squash $@ && git rebase -i --autosquash $REV^' -

usage

$ git commit -am 'bad commit'

$ git commit -am 'good commit'

# Stage changes to correct the bad commit
$ git add .

# Fixup the bad commit. HEAD^ can be replaced by the SHA of the bad commit
$ git fixup HEAD^

You can even skip a step for fixup if you set the git-editor to true:
fixup = !sh -c 'REV=$(git rev-parse $1) && git commit --fixup $@ && GIT_SEQUENCE_EDITOR=true git rebase -i --autosquash $REV^' -

Another useful addition is --autostash (to rebase), so the command will not be interrupted when you got unstaged changes