Reference to a private MR in a public issue can be seen by anyone

While trying to reproduce #6058 (closed) I discovered this issue:

• I mentioned https://gitlab.com/gitlab-org/gitlab-ce/issues/6058 in a private MR rymai/dotfiles!1 (from a private project of mine, fortunately this is not the plan for colonizing Mars);
• A reference to this private issue was created in #6058 (closed).
  • This reference leaks the name of the project as well as the issue title.
  • The reference is not linked when user doesn't have the right to navigate to it and even if he visits the issue URL, he gets redirected to the sign-in screen.

To demonstrate the problem, I will mention this Issue in my private MR.