gofmt: gofmt test harness bug fix

Don't use the rewrite rule from a previous test
for the next test if there is no rewrite rule
provided.

R=r, r2
CC=golang-dev
https://golang.org/cl/4419045

src/cmd/gofmt/rewrite.go

@@ -19,6 +19,7 @@ import (
 
 func initRewrite() {
 	if *rewriteRule == "" {
+		rewrite = nil // disable any previous rewrite
 		return
 	}
 	f := strings.Split(*rewriteRule, "->", -1)