Treediff view switching between Unified and Side by Side loses the selected file

[chrismcband]

Switching modes loses the active file, it shows the first in the list instead.

[NikhilVerma]

@chrismcband This might not be that easy to fix at this moment. We need to do a full page reload to switch between modes and the current way we preserve selection is through hashes in the URL. We might address this at some point in the future.