Add git hook that prevents key leaking

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Dev: https://dev.gitlab.org/gitlab/gitlab-ee/issues/342

Dmitriy

https://gist.github.com/stevendanna/4db970716cabb8f5def4

Based on Sytse's suggestion on: https://dev.gitlab.org/gitlab/blog-posts/issues/71#note_56942;

"can we add https://gist.github.com/stevendanna/4db970716cabb8f5def4 to the git hooks in EE?"

cc/ @sytses @skyruler @dzaporozhets @JobV