Stash-pullrequest-builder-plugin requires that the user who configures a job types in the Git URL (e.g. `ssh://git@example.com/` prefix) to do the actual checkouts. It would be convenient if this value was also among the pre-set variables - or maybe two, for the http(s) and for git+ssh checkouts, if the latter can be requested from Stash. We certainly have http(s) "Stash hostname" for REST API usage, so that variable is easy to re-publish so it can be used in Git repository checkout configuration.