remotes: add support to clone from Mercurial repositories over ssh This commit adds support to clone a remote Mercurial repository over ssh. Interactive password authentication is not implemented, nor is support for pbulic key authentication with passphrases; the repository should be accessible using bare ssh key authentication. For this reason, the ssh options -oBatchMode=yes and -oIdentitiesOnly=yes are added to the ui.ssh setting of Mercurial.