Fix #247 - sh.cwd() not updating cwd on success, with test.

This is so gross of a bug, I'm fixing it on my holidays :) Thanks for finding it, Scott.

The problem is that we're not using the right `cwd` (scope context vs. `this` context).  It will do the right thing with relpaths that contain `./` or `../` but not when there's no prefix.  Easy fix.