enhance: reset diff view context for empty commit

Signed-off-by: leo <longshuang@msn.cn>

src/ViewModels/CommitDetail.cs

@@ -686,6 +686,9 @@ namespace SourceGit.ViewModels
                     {
                         Changes = changes;
                         VisibleChanges = visible;
+
+                        if (visible.Count == 0)
+                            SelectedChanges = null;
                     });
                 }
             });