Called when a stacked branch is activating its fallback locations.
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transform_fallback_location is called with (branch, url), and should
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return a new url. Returning the same url allows it to be used as-is,
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returning a different one can be used to cause the branch to stack on
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a closer copy of that fallback_location. Note that the branch cannot
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have history accessing methods called on it during this hook because
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the fallback locations have not been activated. When there are
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multiple hooks installed for transform_fallback_location, all are
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called with the url returned from the previous hook.The order is
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however undefined.