[BUG?] am --abort on null cache entries fails

Hi,

I was trying git am -3 with a patch that touched files that didn't exist in the branch I was on. Obviously it failed badly, so I wanted to abort out of the git am state with git am --abort. Unfortunately, it seems that git am --abort in this scenario fails with this error:

error: cache entry has null sha1: <non-existant-file>

and then leaves the file in my working tree untracked. This didn't used to happen, so I bisected it down to this commit

commit 4337b5856f88f18da47c176e3cbc95a35627044c
Author: Jeff King <peff@peff.net>
Date: Sat Jul 28 11:05:24 2012 -0400
do not write null sha1s to on-disk index

Which definitely introduced that error message. How do we fix this?