regression in multi-threaded git-pack-index

We have uncovered a regression in this commit:

b8a2486f1524947f232f657e9f2ebf44e3e7a243

The symptom is that 'git fetch' dies with:

error: index-pack died of signal 10
fatal: index-pack failed

I have only been able to reproduce it on a Mac thus far; will try ubuntu next.  We can make it go away by running:

git config pack.threads 1

To reproduce it, download this working copy:

http://commondatastorage.googleapis.com/chromium-browser-snapshots/tmp/src.git.tar.gz

Then:

tar xvfz src.git.tar.gz
cd src.git
git fetch origin refs/heads/lkgr

(That is the shortest reproduction I could come up with; sorry).

Thanks,

Stefan