Issue when changing staged files in a pre-commit hook

I am having an issue where the default commit message is not changed
when adding staged files in a pre-commit hook.
Please see http://stackoverflow.com/questions/34492779/git-update-git-status-part-of-commit-message-after-pre-commit-hook
for details.

Is there a workaround / proper way to achieve what I am trying to do,
or is this a bug?

Regards,
Niek van der Kooy