Add a ref selector to the blame view

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Summary

The blame view doesn't currently have a ref selector. We should add a ref selector as it improves the UX of the page.

See: !195369 (comment 2590697774)

rendering the ref selector would be good for UX because it:

• Allows users to switch between branches to view the Blame across branches
• More consistent with the blob view and other repository pages

blob	blame (current)