Compare IPullRequestModel by ref in setter.

PullRequestModel overrides Equals such that two PRs with the same number are
considered equal. This was causing the Model not to be updated on refresh: we need to use ReferenceEquals. Fixes #901

Author: grokys

src/GitHub.App/ViewModels/PullRequestDetailViewModel.cs

@@ -113,7 +113,18 @@ public PullRequestDetailViewModel(
         public IPullRequestModel Model
         {
             get { return model; }
-            private set { this.RaiseAndSetIfChanged(ref model, value); }
+            private set
+            {
+                // PullRequestModel overrides Equals such that two PRs with the same number are
+                // considered equal. This was causing the Model not to be updated on refresh:
+                // we need to use ReferenceEquals.
+                if (!ReferenceEquals(model, value))
+                {
+                    this.RaisePropertyChanging(nameof(Model));
+                    model = value;
+                    this.RaisePropertyChanged(nameof(Model));
+                }
+            }
         }
 
         /// <summary>