Renamed files show excessive diffs

[FFdhorkin]

This was tested on Codesteam 10.7 on both Windows and OSX.

Repro steps:

1. rename an existing file
2. open a PR
3. open file diff in codestream

Results:

• entire contents of file are shown as diff, even though outside the filename, very little changed. Github does not have this problem - it only shows the changed lines