#7 introduced git submodule support, but the refactor to use git-validate appears to have lost that feature. I opened a PR to reintroduce submodule support in git-validate here. Simply bumping git-validate afterward will reintroduce submodule support in precommit-hook.
We've been using a fork of precommit-hook pointing to that fork of git-validate for six months in several repos without issues.
Thanks1