Remove Repository#lookup and unreachable rugged code

What does this MR do?

Removes the Gitlab::Git::Repository#lookup method, which is a thin wrapper around Rugged. Anything that uses this method relies directly on Rugged, which is wrong because we are removing Rugged from all non-test code in gitlab-ce/ee.

This change also removes some rugged-based methods that are no longer reachable from gitlab-ce/ee.

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Why was this MR needed?

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Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Conform by the code review guidelines
  • Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • Has been reviewed by a Backend maintainer
  • Has been reviewed by a Database specialist
• Conform by the merge request performance guides
• Conform by the style guides
• Conform by the database guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• End-to-end tests pass (package-and-qa manual pipeline job)

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