Expose internal repository create method for gitaly

Closed Jacob Vosmaer (GitLab) requested to merge create-repository-internal into master

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 https://gitlab.com/gitlab-org/gitaly/merge_requests/361.

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Edited by Jacob Vosmaer (GitLab)