Export branches attribute of repository resource

In integrations#892, the newly-added `branches` attribute was exported for the data
source, but not the resource. This exports `branches` in the resource
too.

github/resource_github_repository.go

@@ -126,6 +126,22 @@ func resourceGithubRepository() *schema.Resource {
 				Type:     schema.TypeBool,
 				Optional: true,
 			},
+			"branches": {
+				Type:     schema.TypeList,
+				Computed: true,
+				Elem: &schema.Resource{
+					Schema: map[string]*schema.Schema{
+						"name": {
+							Type:     schema.TypeString,
+							Computed: true,
+						},
+						"protected": {
+							Type:     schema.TypeBool,
+							Computed: true,
+						},
+					},
+				},
+			},
 			"pages": {
 				Type:     schema.TypeList,
 				MaxItems: 1,