don't clobber PWD in wd_show

[kelbyers]

Resolves #114

use a local variable within wd_show to check if we are in a warp point directory or not

❯ wd list
 * All warp points:
control-deploy  ->  ~/git/ose-automation-puppet/control-deploy
          dope  ->  ~/git/one-thd/dope
       ls-base  ->  ~/git/one-thd/locksmith

V6YNJVV4QCMBP in ~/git
❯ wd dope

V6YNJVV4QCMBP in git/one-thd/dope
❯ wd show
 * 1 warp point(s) to current directory: dope

V6YNJVV4QCMBP in git/one-thd/dope
❯ cd ..

V6YNJVV4QCMBP in ~/git/one-thd
❯ wd show
 * No warp point to ~/git/one-thd

V6YNJVV4QCMBP in ~/git/one-thd
❯ echo $PWD
/Users/jxb2016/git/one-thd