Vendor the gitlab-git-test repo

What does this MR do?

Vendor the gitlab-git-test bare Git repository into Git.

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

It is very difficult to make changes to the gitlab-git-test repository without causing lots of failed builds. This change resolves that problem.

The downside is that we are committing binary data into our tree. This MR brings in about 400 KB of compressed data.

The concrete reason for fixing this problem is that the Gitaly team wants to be able to add test cases to spec/lib/gitlab/git.

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support 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 it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/32133