This fixes two problems:
- Set the title ref to the commit description (like
SwitchToDiffFilesController.enter does), rather than just the hash
- SetTitleRef only sets the title of the DynamicTitleBuilder, but doesn't set it
on the view; this only happens in ContextMgr.Activate, so you'd have to switch
to a different side panel and then back to see the new title.