set current branch properly in TAStudio movie load

apparently, GoToFrame might frameadvance, which in turn calls UpdateAfter(), which in turns updates the current selected branch in the TasSession with the value of the current movie.
So, we need to set the current movie's current branch from the session's value before it gets deleted.

- closes #3968

src/BizHawk.Client.Common/movie/tasproj/TasMovie.IO.cs

@@ -134,6 +134,7 @@ namespace BizHawk.Client.Common
 				try
 				{
 					TasSession = JsonConvert.DeserializeObject<TasSession>(json);
+					Branches.Current = TasSession.CurrentBranch;
 				}
 				catch
 				{

src/BizHawk.Client.EmuHawk/tools/TAStudio/TAStudio.cs

@@ -558,7 +558,6 @@ namespace BizHawk.Client.EmuHawk
 			SetUpToolStripColumns();
 
 			CurrentTasMovie.PropertyChanged += TasMovie_OnPropertyChanged;
-			CurrentTasMovie.Branches.Current = CurrentTasMovie.TasSession.CurrentBranch;
 			BookMarkControl.UpdateTextColumnWidth();
 			MarkerControl.UpdateTextColumnWidth();
 			// clear all selections