windows: reset command var to sh for correct error output messages
windows: reset command var to sh for correct error output messages

R=golang-dev, r
CC=golang-dev
http://codereview.appspot.com/4492043

Committer: Rob Pike <4dc7c9ec434ed06502767136789763ec11d2c4b7@golang.org>

tav/go,tav/go,tav/go,tav/go,tav/go,tav/go,tav/go