fetch: move `Component::from` outside of loop — heartwood

The previous code would convert the remote into a Component on each iteration of the loop.

The conversion now happens outside of the loop and can be reused.

         &'a self,
         remote: &'a PublicKey,
     ) -> impl Iterator<Item = (RefString, Oid)> + 'a {
         let remote = Component::from(remote);
         self.0.iter().filter_map(move |(refname, oid)| {
             let ns = refname.to_namespaced()?;
             (ns.namespace() == Component::from(remote))
                 .then(|| (ns.strip_namespace().to_ref_string(), *oid))
             (ns.namespace() == remote).then(|| (ns.strip_namespace().to_ref_string(), *oid))
         })
     }