checkout @{-1} bug/oddity

In case this is worth fixing:

  | % g co -b foo
  | Switched to a new branch 'foo'
  | 
  | % g co -
  | Switched to branch 'master'
  | 
  | % g co -b bar
  | Switched to a new branch 'bar'
  | 
  | % g co bar
  | Already on 'bar'
  | 
  | % g co -
  | Already on 'bar'

In my opinion the last command should switch to master, as in, switching
to the current branch should not lose the @{-1} history.

Thanks,

-- 
- Are you sure we're good?
- Always.
        -- Rory and Lorelai