optimize<CreateBranch>: 增加一个对于空仓库建立分支的错误提示

src/Resources/Locales/en_US.xaml

@@ -96,6 +96,7 @@
     <sys:String x:Key="Text.CreateBranch.LocalChanges.StashAndReply">Stash &amp; Reapply</sys:String>
     <sys:String x:Key="Text.CreateBranch.LocalChanges.Discard">Discard</sys:String>
     <sys:String x:Key="Text.CreateBranch.Checkout">Check out after created</sys:String>
+    <sys:String x:Key="Text.CreateBranch.Idle">Git do not hold any branch until you do first commit.</sys:String>
 
     <sys:String x:Key="Text.CreateTag">Create Tag</sys:String>
     <sys:String x:Key="Text.CreateTag.BasedOn">New Tag At :</sys:String>

src/Resources/Locales/zh_CN.xaml

@@ -96,6 +96,7 @@
     <sys:String x:Key="Text.CreateBranch.LocalChanges.StashAndReply">贮藏并自动恢复</sys:String>
     <sys:String x:Key="Text.CreateBranch.LocalChanges.Discard">忽略</sys:String>
     <sys:String x:Key="Text.CreateBranch.Checkout">完成后切换到新分支</sys:String>
+    <sys:String x:Key="Text.CreateBranch.Idle">对于空仓库，只有提交一次有效数据，Git 才会创建第一个分支</sys:String>
 
     <sys:String x:Key="Text.CreateTag">新建标签</sys:String>
     <sys:String x:Key="Text.CreateTag.BasedOn">标签位于 ：</sys:String>

src/Views/Widgets/Dashboard.xaml.cs

@@ -401,7 +401,11 @@ namespace SourceGit.Views.Widgets {
 
         private void OpenNewBranch(object sender, RoutedEventArgs e) {
             var current = repo.Branches.Find(x => x.IsCurrent);
-            if (current != null) new Popups.CreateBranch(repo, current).Show();
+            if (current != null) { 
+                new Popups.CreateBranch(repo, current).Show();
+            } else {
+                Models.Exception.Raise(App.Text("CreateBranch.Idle"));
+            }
             e.Handled = true;
         }