git diff-index and both deleted conflict

Hello all,

Could you advise me the fastest way to check in my script 
that there's "no local changes in the repository (in the working tree and 
index)" so that I could perform "git rebase" there?

git svn dcommit" should check for the same because it performs rebase.
"git svn dcommit" calls "git diff-index HEAD".

According to the documentation "git diff-index HEAD" performs the check I need. 
But I've discovered that in the case of "both deleted" conflict "git diff-index 
HEAD" outputs nothing but rebase still doesn't want to run. 

So, at least either "git diff-index HEAD" or its documentation and "git-svn" 
should be fixed, I think.

And I'm still asking for the fastest method that could predict that working 
tree and index are clean, no conflict, and I can run "git rebase".


A simple script to generate "both deleted":

#!/bin/sh

git init
touch foo
git add foo
git commit -m 'initial commit'
git checkout -b tmp
git mv foo X
git commit -m 'rename to X'
git checkout -
git mv foo Y
git commit -m 'rename to Y'
git merge tmp
git add Y
git rm X