gofix: fix diff regression from exec change

Also pass -u to diff to be consistent with gofmt.

Fixes #1619

R=golang-dev, gri
CC=golang-dev
https://golang.org/cl/4591041

src/cmd/gofix/main.go

@@ -248,5 +248,11 @@ func diff(b1, b2 []byte) (data []byte, err os.Error) {
 	f1.Write(b1)
 	f2.Write(b2)
 
-	return exec.Command("diff", f1.Name(), f2.Name()).CombinedOutput()
+	data, err = exec.Command("diff", "-u", f1.Name(), f2.Name()).CombinedOutput()
+	if len(data) > 0 {
+		// diff exits with a non-zero status when the files don't match.
+		// Ignore that failure as long as we get output.
+		err = nil
+	}
+	return
 }