os: fix SameFile to work for directories on windows

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/5756064

src/pkg/os/stat_windows.go

@@ -199,7 +199,7 @@ func (s *winSys) loadFileId() error {
 	}
 	s.Lock()
 	defer s.Unlock()
-	h, e := syscall.CreateFile(syscall.StringToUTF16Ptr(s.path), syscall.GENERIC_READ, syscall.FILE_SHARE_READ, nil, syscall.OPEN_EXISTING, 0, 0)
+	h, e := syscall.CreateFile(syscall.StringToUTF16Ptr(s.path), 0, 0, nil, syscall.OPEN_EXISTING, syscall.FILE_FLAG_BACKUP_SEMANTICS, 0)
 	if e != nil {
 		return e
 	}

src/pkg/path/filepath/path_test.go

@@ -666,10 +666,6 @@ var absTests = []string{
 }
 
 func TestAbs(t *testing.T) {
-	if runtime.GOOS == "windows" {
-		t.Log("TestAbs disabled on windows")
-		return
-	}
 	oldwd, err := os.Getwd()
 	if err != nil {
 		t.Fatal("Getwd failed: ", err)