refactor/rename: skip TestDiff on plan9, whose diff has no -u

Change-Id: I372591ad3f2804dcd20f0ab3b5a91ddda762cba8
Reviewed-on: https://go-review.googlesource.com/33586
Reviewed-by: Robert Griesemer <gri@golang.org>

refactor/rename/rename_test.go

@@ -1130,6 +1130,9 @@ func TestDiff(t *testing.T) {
 	if runtime.GOOS == "windows" {
 		t.Skipf("diff tool non-existent for windows on builders")
 	}
+	if runtime.GOOS == "plan9" {
+		t.Skipf("plan9 diff tool doesn't support -u flag")
+	}
 
 	defer func() {
 		Diff = false