gofrontend: add codereview.cfg

Set issuerepo such that issue references are rewritten to the golang/go
repo by git-codereview.

See CL 4131 for reference.

Change-Id: I06207045fb92a2764f22e9a3fd374f2fdb7ccaca
Reviewed-on: https://go-review.googlesource.com/c/152558
Reviewed-by: Than McIntosh <thanm@google.com>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

codereview.cfg

@@ -0,0 +1 @@
+issuerepo: golang/go