git reset --soft HEAD^
  # make changes so index only contains what you want
  git commit -a -C ORIG_HEAD

of if you prefer:

  git config --global alias.undo reset --soft HEAD^
  git config --global alias.redo commit -a -C ORIG_HEAD

then:

  git undo
  # make changes so index only contains what you want
  git redo

You could use -c (--reedit-message=) instead of -C (--reuse-message=) for further flexibility.

ORIG_HEAD is set to point to the most recent HEAD when using reset. See "man git-reset" > EXAMPLES > "Undo a commit and redo".