When a S3 remote uses exporttree=yes versioning=yes public=yes,
it's possible to use anonymous http to download anything from it. git-annex
does not yet support that, nor does whereis show the urls.

Should not be super hard to add, but it involves converting `getpublicurl`
into an Annex action and distinguishing between different uses of it,
some of which work with this and some don't. --[[Joey]]

> [[done]] --[[Joey]]
