Provide host name to certificate_check_cb

Verifying a certificate also requires the hostname of the host we're connecting to, thus, provide host name to certificate_check_cb
Your're right, there's no redirect in ssh. However, as an application using libgit2 I just want to pass a URI to libgit2 and don't want to bother about hostnames or parsing the URI to get the hostname (as libgit2 does not export the URI parser I would have to write my own).

Giving the URI to the certificate check callback has lots of advantages for applications using libgit2, e.g. you can use it show it directly do your users or to store the checked ssh host key for later automatic acceptance.