clfilter: introduce a `hassecret` function
We can only use copy clone if the cloned repo do not have any secret changeset.
The current method for that is to run the "secret()" revset on the remote repo.
But with proper filtering of hidden or unserved revision by the remote this
revset won't return any revision even if some exist remotely. This changeset
adds an explicit function to know if a repo have any secret revision or not.
The other option would be to disable filtering for the query but I prefer the
approach above, lighter both regarding code and performance.