Add the ahead case

Author: junichi11

src/main/java/com/junichi11/netbeans/modules/github/issues/issue/GitHubIssueController.java

@@ -665,6 +665,8 @@ public void propertyChange(PropertyChangeEvent evt) {
                         panel.setErrorMessage(Bundle.ComparePullRequestPropertyChangeListener_message_conflict());
                         descriptor.setValid(true);
                         return;
+                    case "ahead": // NOI18N
+                        break;
                     default:
                         break;
                 }