When selecting a range of commits where the first commit adds a file and the later commit(s) change it, the `is_new_file` state was being incorrectly computed as `True`. In the case where we have a base filediff, we instead should be treating it as an existing file rather than a new one. Testing Done: Loaded a commit range that was previously triggering the bug (especially noticeable with the way binary files were loaded). Saw that I was now seeing it treated as an existing file rather than new.