cmd/go-contrib-init: match subdomain-less .gitcookies file

My `.gitcookies` file starts with ".googlesource.com", which
errored because it did not match "go.googlesource.com" or
"go-review.googlesource.com". Fix this by optionally matching on
"go.googlesource.com" or ".googlesource.com".

Fixes golang/go#20992.

Change-Id: I29d3c0b1e958382495a90502f280bdb52868c2c7
Reviewed-on: https://go-review.googlesource.com/48230
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
diff --git a/cmd/go-contrib-init/contrib.go b/cmd/go-contrib-init/contrib.go
index 112ecfe..cb96453 100644
--- a/cmd/go-contrib-init/contrib.go
+++ b/cmd/go-contrib-init/contrib.go
@@ -18,6 +18,7 @@
 	"os"
 	"os/exec"
 	"path/filepath"
+	"regexp"
 	"runtime"
 	"strings"
 )
@@ -62,13 +63,14 @@
 	return "go"
 }
 
+var googleSourceRx = regexp.MustCompile(`(?m)^(go|go-review)?\.googlesource.com\b`)
+
 func checkCLA() {
 	slurp, err := ioutil.ReadFile(cookiesFile())
 	if err != nil && !os.IsNotExist(err) {
 		log.Fatal(err)
 	}
-	if bytes.Contains(slurp, []byte("go.googlesource.com")) &&
-		bytes.Contains(slurp, []byte("go-review.googlesource.com")) {
+	if googleSourceRx.Match(slurp) {
 		// Probably good.
 		return
 	}