Fix broken repo errors in the UI

This should prevent repo errors (or 404s) in the UI, together with https://gitlab.com/gitlab-org/gitlab_git/merge_requests/124

The exists? cache is now expired if the repo gets broken.

• Update Gemfile to use latest gitlab_git with the fix from https://gitlab.com/gitlab-org/gitlab_git/merge_requests/124
• CHANGELOG entry added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together

Related MR: https://gitlab.com/gitlab-org/gitlab_git/merge_requests/124

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/20501