gofmt: fix -d regression from exec change
gofmt: fix -d regression from exec change

Fixes issue 1916

R=golang-dev, gri
CC=golang-dev
http://codereview.appspot.com/4590041
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go