Editing the root commit

I've recently been polishing up some private code for release, using git rebase --interactive to expand on some commit messages.

In a couple of cases, I wanted to edit the root commit. (Adding a COPYING file, for example.) I've been successfully doing this with an explicit

  ROOT=$(git log --pretty=format:%H | tail -n 1)
  git checkout $ROOT
  git commit --amend --message='Initial import modified' # for example
  git rebase --onto HEAD $ROOT master

However, this brought two questions to mind.

The first is whether there's a clever symbolic way to refer to the root of the current branch, rather than tailing git log output? gitrevisions(7) doesn't obviously suggest one.

The second question is whether it's possible to use git rebase --interactive to edit the root commit along with some subsequent ones in one fell swoop?

My fingers half-remember doing something like

  git checkout --orphan rewritten
  git rm -rf
  git rebase --interactive --root --onto rewritten master

a year or so ago, but this now fails (on git 1.7.10) with the somewhat surprising error

  $ git rebase --root --onto rewritten master
  fatal: Needed a single revision
  Does not point to a valid commit: master

Am I misremembering the recipe here, or has the behaviour changed? It seems to fail identically with or without --interactive.

Best wishes,

Chris.