break out repo split and add unit test

cmd/drone-docker-ecr/main.go

@@ -55,7 +55,7 @@ func main() {
 	}
 
 	if create {
-		err = ensureRepoExists(svc, strings.SplitN(repo, "/", 2)[1])
+		err = ensureRepoExists(svc, getRepoName(repo))
 		if err != nil {
 			log.Fatal(fmt.Sprintf("error creating ECR repo: %v", err))
 		}
@@ -75,6 +75,10 @@ func main() {
 	}
 }
 
+func getRepoName(repo string) string {
+	return strings.SplitN(repo, "/", 2)[1]
+}
+
 func ensureRepoExists(svc *ecr.ECR, name string) (err error) {
 	input := &ecr.CreateRepositoryInput{}
 	input.SetRepositoryName(name)