Avoid a stack overflow on ICommand executions

The implementation for OleMenuCommand apparently doesn't protect against
reentrant calls of the canEnable callbacks (same bug ReactiveUI has, but
at least there the class is private...). When executing a command, if
the execution triggers a xaml refresh, it will call canEnable, re-enable
the buttons, then execute all over again! As expected, this doesn't work
well...

CanEnable callbacks need to return false until the execution is fully
done.

Author: shana

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

@@ -58,23 +58,23 @@ public override void Initialize(IServiceProvider serviceProvider)
                 (s, e) => Reload(new ViewWithData { Flow = UIControllerFlow.PullRequests, ViewType = UIViewType.PRList }));
 
             back = ServiceProvider.AddCommandHandler(GuidList.guidGitHubToolbarCmdSet, PkgCmdIDList.backCommand,
-                () => currentNavItem > 0,
+                () => !disabled && currentNavItem > 0,
                 () => {
                     DisableButtons();
                     Reload(navStack[--currentNavItem], true);
                 },
                 true);
 
             forward = ServiceProvider.AddCommandHandler(GuidList.guidGitHubToolbarCmdSet, PkgCmdIDList.forwardCommand,
-                () => currentNavItem < navStack.Count - 1,
+                () => !disabled && currentNavItem < navStack.Count - 1,
                 () => {
                     DisableButtons();
                     Reload(navStack[++currentNavItem], true);
                 },
                 true);
 
             refresh = ServiceProvider.AddCommandHandler(GuidList.guidGitHubToolbarCmdSet, PkgCmdIDList.refreshCommand,
-                () => navStack.Count > 0,
+                () => !disabled && navStack.Count > 0,
                 () => {
                     DisableButtons();
                     Reload();
@@ -224,10 +224,12 @@ void UpdateToolbar()
             back.Enabled = currentNavItem > 0;
             forward.Enabled = currentNavItem < navStack.Count - 1;
             refresh.Enabled = navStack.Count > 0;
+            disabled = false;
         }
 
         void DisableButtons()
         {
+            disabled = true;
             back.Enabled = false;
             forward.Enabled = false;
             refresh.Enabled = false;
@@ -279,6 +281,8 @@ public bool IsGitHubRepo
         public bool IsShowing => true;
 
         bool disposed = false;
+        private bool disabled;
+
         protected override void Dispose(bool disposing)
         {
             if (disposing)