BUG: Wrong "global session" label when saving tab session for the first time.

The problem is that xolox#session#get_label() determines the tab-scope through xolox#session#is_tab_scoped(), which checks the existence of t:this_session. But before the save of a tab-scoped session, this variable hasn't been set yet, and therefore it yields the wrong label "global session"!

All places that invoke xolox#session#get_label() in fact already determine the scope themselves, so let's scrap the logic and just pass in the boolean flag as an additional is_tab_scoped argument. This eliminates any risk that the message contradicts what's actually happening, and it's simpler, too.
Merge pull request #51

Pull request #51 on GitHub:
  https://github.com/xolox/vim-session/pull/51