Fix g:CommandTTraverseSCM='pwd' staleness
Fix g:CommandTTraverseSCM='pwd' staleness

In fixing https://github.com/wincent/command-t/issues/108, I introduced
another bug: https://github.com/wincent/command-t/issues/109, and in
fixing that I introduced: https://github.com/wincent/command-t/issues/118

Specifically, under 'pwd' mode we were re-using the `@path` value and
never resetting it. Try to make this more obvious and robust, at the
expense of some repetition.

besarthoxhaj/command-t,vrkansagara/command-t,mtimkovich/command-t,vrkansagara/command-t,yrro/command-t,wincent/command-t,supriyantomaftuh/command-t,wincent/command-t,lencioni/command-t,supriyantomaftuh/command-t,lencioni/command-t,simichaels/dotfiles,yrro/command-t,1234-/command-t,besarthoxhaj/command-t,wincent/command-t,1234-/command-t,simichaels/dotfiles,mtimkovich/command-t