As of the release of 2.18.0, $GIT_DIR is no longer set before calling pre-commit hooks. This change was introduced in "set_work_tree: use chdir_notify" (8500e0de) and is still present in master.
I reviewed the discussion when this change was initially submitted, and I don't think this behavior change was intentional. I have several hooks that were broken like this, and from a quick Google search, using $GIT_DIR without setting a default does seem pretty common in hooks. Greg
