how to create an empty commit under an existing initial commit

gistfile1.sh

 $ rm -rf .git
 $ git init
Initialized empty Git repository in /Users/timcharper/project/.git/
 $ date > file.txt
 $ git add file.txt
 $ git commit -m "initial commit"
[master (root-commit) 399a71c] initial commit
 1 files changed, 1 insertions(+), 0 deletions(-)
 create mode 100644 file.txt
 master$ 
 master$ git symbolic-ref HEAD refs/heads/newbranch
 $ git rm --cached *
rm 'file.txt'
 $ git status
# On branch newbranch
#
# Initial commit
#
# Changes to be committed:
#   (use "git rm --cached <file>..." to unstage)
#
#	new file:   file.txt
#
 $ git rm --cached *
rm 'file.txt'
 $ git status
# On branch newbranch
#
# Initial commit
#
# Untracked files:
#   (use "git add <file>..." to include in what will be committed)
#
#	file.txt
nothing added to commit but untracked files present (use "git add" to track)
 $ git commit -m "empty initial commit" --allow-empty
[newbranch (root-commit) d3e31fe] empty initial commit
 newbranch$ git checkout master
error: Untracked working tree file 'file.txt' would be overwritten by merge.
 newbranch$ git clean -f
Removing file.txt
 newbranch$ git checkout master 
Switched to branch 'master'
 master$ git rebase newbranch 
First, rewinding head to replay your work on top of it...
Applying: initial commit
 master$ git log
commit 814c310abf3be91c8e137db3ca83e294a0d9099b
Author: Tim Harper <[email protected]>
Date:   Wed Sep 30 05:39:25 2009 -0600

    initial commit

commit d3e31fed9490a5dc3e095975b4bfa486d1fc2679
Author: Tim Harper <[email protected]>
Date:   Wed Sep 30 05:42:18 2009 -0600

    empty initial commit