Very edge case ActivityLog.xml dialog in InlineRename

[DavidKarlas]

Version Used:
Any.

Steps to Reproduce:

1. Start renaming
2. Make multi-selection
3. Type something

Expected Behavior:
No exception.
Actual Behavior:
Exception happen:

System.InvalidOperationException: Sequence contains more than one element
   at System.Linq.Enumerable.Single[TSource](IEnumerable`1 source)
   at Microsoft.CodeAnalysis.Editor.Implementation.InlineRename.InlineRenameSession.OpenTextBufferManager.OnTextBufferChanged(Object sender, TextContentChangedEventArgs args)
   at Microsoft.VisualStudio.Text.Utilities.GuardedOperations.RaiseEvent[TArgs](Object sender, EventHandler`1 eventHandlers, TArgs args)
--- End of stack trace from previous location where exception was thrown ---
   at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
   at Microsoft.VisualStudio.Telemetry.WindowsErrorReporting.WatsonReport.GetClrWatsonExceptionInfo(Exception exceptionObject)

I found this by looking at code... It's super unlikely user would do multiselection while renaming variable...

[sharwell]

Duplicate of #34903

[sharwell]

Decided not to make this a duplicate since this one is much easier to resolve than #34903