[BUG?] "error: cache entry has null sha1"

This is totally unrelated to this thread, but I am seeing a strange error from the "let's make sure we won't write null sha1" code we added recently. This reproduces reliably for me, even in a freshly cloned repository without previous rerere records:

 - Check out v1.8.4.1

   $ git checkout v1.8.4.1^0

   at this point your HEAD should be detached.

 - Apply the first patch from this series

   $ git am -3 20131031063530.GA5812@sigill.intra.peff.net

 - Apply the second patch from this series

   $ git am -3 20131031063626.GB5812@sigill.intra.peff.net

   Up to this point things should advance smoothly

 - Attempt to apply the second patch _again_.

   $ git am -3 20131031063626.GB5812@sigill.intra.peff.net

   This should detect a no-op, but somehow leaves things in a
   conflicted state.  I haven't looked into what is wrong, but this
   message is not about this failure.

 - Try to discard

   $ git am --abort
   error: cache entry has null sha1: remote-curl.c
   fatal: unable to write new index file

   This should not happen, no?

"git reset --hard" will remove the funnies, but still...