git-diff-tree does not use alternate objects for submodules

I have a project with a submodule. Both have objects/info/alternate (different ones).

After running 'git gc', running gitk on the superproject results in:
  Submodule sub 227e2b5...d8597e2 (commits not present)

This is the git-diff-tree command that gitk runs:
  git diff-tree -r -p --textconv --submodule -C --cc --no-commit-id
-U6 --root d117679ef4a4fb4143401d3a4bc4a484c9bf76ff

I'm using msysGit 1.7.10

- Orgad