Create wiki repo if missing during HTTP clone

What does this MR do?

Older GitLab installations can contain projects that have the wiki feature enabled, yet do not have a wiki repository on disk.

These repositories are causing a flood of 'Not Found' errors as we replicate them with Geo to a gitlab.com secondary.

This change creates an empty wiki repository for the project if it doesn't already exist. I also tried just returning a 404 but this approach seemed much easier to integrate into the existing code.

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

Why was this MR needed?

To get crazy git clone error rates down on gitlab.com.

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
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

Closes gitaly#1031 (closed)

Closes https://gitlab.com/gitlab-com/infrastructure/issues/3734