"fatal: git-write-tree: error building trees" from `git stash`

  Heya,
I just ran into the following with `git stash`.  The set-up:

        git init
        echo "Initial" > foo
        git add .
        git commit -m 'Initial commit'
        echo "Rewrite" > foo
        git commit -am 'Second commit, rewrites content'
        echo "Stashed changes" >> foo
        git stash
        git co HEAD~
        
$ git stash pop
Auto-merging foo
CONFLICT (content): Merge conflict in foo
Recorded preimage for 'foo'

$ git stash foo: needs merge foo: needs merge foo: unmerged (aeaa7e5e87cf309a7368d5d92a71c1f9e6a8c9e7) foo: unmerged (a77fa514de2720c72c1a861de098595959a2c97a) foo: unmerged (4a622d2b991f1a19ba7be313a46dc6f03692cd0a) fatal: git-write-tree: error building trees Cannot save the current index state

 - Alex