Consider reexposing GL_ID to custom hooks

In our company, we use a custom git hook to perform additional permission checks based on the user that is pushing (essentially, branch based access control), using GL_ID to identify the user that is performing the action. This broke in GitLab 8.10 when commit 48d292eb removed GL_ID from the environment before calling the custom hook.

Could you consider re-adding this, or to expose the identified user id / user name to the custom hooks? Currently, we have to use the internal api to identify which user an ssh key belongs to, because to our knowledge, the public API does not even allow this for admins.