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

tv42/old-go,dsriram/golang,apparentlymart/golang,apparentlymart/golang,gnanderson/go,dsriram/golang,gnanderson/go,dsriram/golang,tv42/old-go,apparentlymart/golang,tv42/old-go,tv42/old-go,pombredanne/go-deleteme,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,ssrl/go,pombredanne/go-deleteme,tv42/old-go,gnanderson/go,apparentlymart/golang,dsriram/golang,ssrl/go,gnanderson/go,pombredanne/go-deleteme,apparentlymart/golang,ssrl/go,ssrl/go,apparentlymart/golang,ssrl/go,pombredanne/go-deleteme,pombredanne/go-deleteme,dsriram/golang,gnanderson/go,ssrl/go,tv42/old-go,apparentlymart/golang,dsriram/golang,dsriram/golang,gnanderson/go,ssrl/go,pombredanne/go-deleteme