GitHub REST resources typically return full URLs to fetch related
resources at. We used to parse those URLs to find just the path portion
and pass that in to the `REST()` function, which only accepted paths. By
doing so, we are essential de-constructing a URL just to re-assemble it
again. While re-assembling it for Enterprise, though, we would
accidentally inject an extra `api/v3/` prefix where one was not needed.
The solution is just to use raw URLs as reported by the REST API with
no modifications. This extends the `REST()` function to accept full URLs
in addition to just paths to resources.
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