Renamed PR files aren't compared with previous version

[jcansdale]

• GitHub Extension for Visual Studio version: 2.3.7 (dev)
• Visual Studio version: 2015

What happened (with steps, logs and screenshots, if possible)

1. Opened a PR that contains renamed files (PR 1346)
2. Selected View Changes on renamed file
3. The LHS file in diff view is blank:
   

• Expected renamed file to be compared with original (like on github.com):
  
• Expected renamed file to be prefixed with [rename ...] (to be consistent with [add]):
  

This has made reviewing PR 1346 tricky because I can't see what's actually new in the renamed files.