Fix assertion error.

When `CheckoutMode == Push` and has uncommitted changes.

Author: grokys

src/GitHub.App/ViewModels/PullRequestDetailViewModel.cs

@@ -308,7 +308,7 @@ public async Task Load(IPullRequestModel pullRequest)
 
             var clean = await pullRequestsService.IsCleanForCheckout(repository);
 
-            CheckoutDisabledMessage = (!clean && CheckoutMode != CheckoutMode.UpToDate) ?
+            CheckoutDisabledMessage = (!clean && CheckoutMode != CheckoutMode.UpToDate && CheckoutMode != CheckoutMode.Push) ?
                 $"Cannot {GetCheckoutModeDescription(CheckoutMode)} as your working directory has uncommitted changes." :
                 null;