Expose internal repository create method for gitaly

What does this MR do?

Exposes part of Gitlab::Git::Repository.create for use inside gitaly-ruby.

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Why was this MR needed?

We need this internal method in gitaly!361 (merged).

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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
• 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

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