Deprecate repo.Refs.Move() in favor of repo.Refs.Rename()

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• Add the new Rename() method
• Delegate the current Move() implementation to Rename()
• Decorate Move() with an obsolete attribute (using the standard message pattern)
• Modify existing code so Move() is no longer called
  BranchCollection: Obsolete Move and introduce Rename. #738 is a great example of a similar fix

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Fixed in vNext