Assume unchanged for submodule directory can't be undone in TGit

What steps will reproduce the problem?

1. Make a commit in submodule directory
2. Open Check for modifications window on root repository
3. Right click on changed submodule directory
4. Invoke Assume unchanged

What is the expected output? What do you see instead?

Git properties dialog for submodule directory should have a checkbox for Assume unchanged

What version of TortoiseGit and Git are you using? On what operating system?

2.6.0.0