[llvm] Avoid exposing unknown git repositories (PR #105220)

[Kristof Beyls via llvm-commits]

kbeyls wrote:

I added @tstellar and @petrhosek as reviewers. Would one of you be able to help review this? Or do you otherwise have suggestions for who would be good to review this PR?

https://github.com/llvm/llvm-project/pull/105220