Warn if user tries to open solution file that isn't checked out

Stop user from opening solution files that aren't related to the target PR diff file.

Author: jcansdale

src/GitHub.VisualStudio/Views/GitHubPane/PullRequestDetailView.xaml.cs

@@ -132,6 +132,12 @@ async Task DoOpenLiveFile(IPullRequestFileNode file)
         {
             try
             {
+                if (!ViewModel.IsCheckedOut)
+                {
+                    ShowInfoMessage("Checkout PR branch before opening file in solution.");
+                    return;
+                }
+
                 var fullPath = ViewModel.GetLocalFilePath(file);
 
                 var activeView = NavigationService.FindActiveView();
@@ -152,6 +158,13 @@ async Task DoOpenLiveFile(IPullRequestFileNode file)
             }
         }
 
+        static void ShowInfoMessage(string message)
+        {
+            ErrorHandler.ThrowOnFailure(VsShellUtilities.ShowMessageBox(
+                Services.GitHubServiceProvider, message, null,
+                OLEMSGICON.OLEMSGICON_INFO, OLEMSGBUTTON.OLEMSGBUTTON_OK, OLEMSGDEFBUTTON.OLEMSGDEFBUTTON_FIRST));
+        }
+
         async Task DoDiffFile(IPullRequestFileNode file, bool workingDirectory)
         {
             try