Fix session scope on open

BUG: The automatic load of a previous tab scoped session reports _Opened **global** session_.

For the message after opening a session, use the `t:this_session` / `v:this_session` information from the loaded session instead of the current configuration. This matters especially when a session is auto-loaded on Vim startup, because then the scope cannot be determined from the command, (since there is no command).
The `:OpenSession` / `:OpenTabSession` commands actually perform the same (maybe there should be a warning when a global session is opened through `:OpenTabSession`?!)