ranamed file can't be compared

[shui-dun]

procedure to reproduct this bug

1. in a commit, I rename a file named a.py to copy.py, and modified its content
2. I want to view the difference before and after this commit
3. I click the "previous" button, but it make no sense because before this commit copy.py is not exists

expected behavior

1. click the "previous" button
2. show the difference between copy.py and a.py