Add a spec for .escape_for_single_quotes
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.
Update docs for 1.11.3 release