go: specify tools resource by branch name alone (#10097)

If branch name is not specified, git checkout will fail when the
tools resource has previously been cached. If branch name is specified,
the revision will not be used or validated. So only specify branch name.

Formula/go.rb

@@ -8,10 +8,10 @@ class Go < Formula
     version "1.8"
     sha256 "406865f587b44be7092f206d73fc1de252600b79b3cacc587b74b5ef5c623596"
 
-    # revision should be tip of tree commit in release-branch.goX.Y
+    go_version = version.to_s.split(".")[0..1].join(".")
     resource "gotools" do
       url "https://go.googlesource.com/tools.git",
-          :revision => "5682db0e919ed9cfc6f52ac32e170511a106eb3b"
+          :branch => "release-branch.go#{go_version}"
     end
   end