Fix repo existence check in GitAccessWiki

What does this MR do?

The GitAccess and GitAccessWiki classes are the gatekeepers that decide whether a user can push or pull a Git repository via HTTP or SSH. One of the jobs of these classes it to reject access to repositories that do not exist on disk with a user-friendly message and an appropriate error code.

Due to an oversight, the repository existence check in GitAccessWiki was checking the main repository instead of the wiki repository. This change fixes that.

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

Why was this MR needed?

To get rid of the flood of Gitaly errors caused by Geo cloning non-existing wiki repos 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 https://gitlab.com/gitlab-org/gitlab-ce/issues/43385

Will fix https://gitlab.com/gitlab-com/infrastructure/issues/3734 once deployed.