This right there is the what gives me pause. Every time I need to do something out of the box, I need to start googling. Rather than there being an easy to remember command to do it, one that has a good name that indicates what it does; or, more specifically, what one uses it to accomplish. It's like naming a field "list" in code; I know it's a list... but name it so I know what is in it or what it's for.

And to add to the fun, there's multiple other answers in this thread on how to accomplish the same task.

  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".