Set the GL_REPOSITORY env variable on Gitlab::Git::Hook

What does this MR do?

Sets the GL_REPOSITORY env variable on Gitlab::Git::Hook. It takes the chance to reduce code redundancy now that project gets passed around instead of just the repo_path.

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

No

Why was this MR needed?

GL_REPOSITORY is now required on the hooks after https://gitlab.com/gitlab-org/gitlab-shell/merge_requests/136

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 #34502 (closed)