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.

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.