os/exec: make LookPath always search the current directory under Windows.

cmd.exe implicitly looks in "." before consulting PATH.
LookPath should match this behavior.

R=alex.brainman, rsc
CC=golang-dev
https://golang.org/cl/5434093

src/pkg/os/exec/lp_windows.go

@@ -63,11 +63,10 @@ func LookPath(file string) (f string, err error) {
 		}
 		return ``, &Error{file, err}
 	}
-	if pathenv := os.Getenv(`PATH`); pathenv == `` {
 	if f, err = findExecutable(`.\`+file, exts); err == nil {
 		return
 	}
-	} else {
+	if pathenv := os.Getenv(`PATH`); pathenv != `` {
 		for _, dir := range strings.Split(pathenv, `;`) {
 			if f, err = findExecutable(dir+`\`+file, exts); err == nil {
 				return