Fix refresh of PR list switching to "create" form.

Fixes #480. Issue was happening because of an extra PRCreation
item in the nav stack when Refresh was clicked 2nd time.

It was there because when navigating forward for the 2nd Create,
GoForward method wasn't removing everything after the current item.

Author: OlsonAndrewD

src/GitHub.VisualStudio/UI/Views/GitHubPaneViewModel.cs

@@ -279,8 +279,8 @@ void StartFlow(UIControllerFlow controllerFlow, [AllowNull]IConnection conn, Vie
         void GoForward(ViewWithData data)
         {
             currentNavItem++;
-            if (currentNavItem < navStack.Count - 1)
-                navStack.RemoveRange(currentNavItem, navStack.Count - 1 - currentNavItem);
+            if (currentNavItem < navStack.Count)
+                navStack.RemoveRange(currentNavItem, navStack.Count - currentNavItem);
             navStack.Add(data);
         }