Following up on Special remote redirect to URL, it would be useful for CHECKPRESENT (and also CHECKPRESENTEXPORT) to be able to redirect to an url, and let git-annex do the checking.

This will let external special remotes that are readonly and can calculate urls on their own avoid needing to implement HTTP at all.

The protocol extension would be:

EXTENSIONS CHECKPRESENT-URL
CHECKPRESENT-URL Key Url

--Joey

implemented --Joey