git-codereview: fix Go 1.8 t.Helper fix

Don't add a new stack frame.

Thanks to Dominik Honnef.

Change-Id: I817f95fb23e80dcdff733a7b35fa72bac89958f8
Reviewed-on: https://go-review.googlesource.com/80795
Run-TryBot: Brad Fitzpatrick <bradfitz@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Dominik Honnef <dominik@honnef.co>
diff --git a/git-codereview/go19_test.go b/git-codereview/go19_test.go
index 784ec41..c2dc46e 100644
--- a/git-codereview/go19_test.go
+++ b/git-codereview/go19_test.go
@@ -9,5 +9,5 @@
 import "testing"
 
 func init() {
-	setHelper = func(t *testing.T) { t.Helper() }
+	setHelper = (*testing.T).Helper
 }