While debugging another issue, I discovered that if a diff contains a "new file" but has a parent diff that previously deleted that file, it would show in two-column mode with the left-hand side totally empty. This traced back to an old fix done way back in 2012, back when we only had a `source_revision` field which was overloaded between both the diff and parent diff. That was fixed in 3.0.19, but the `is_new_file` calculation still completely turned it off if we had a parent diff present at all. I've changed this so that if we have a modern `parent_source_revision`, we allow ourselves to trust the `newfile` flag. Testing Done: - Verified that this situation now correctly appeared as a new file in the diff viewer. - Ran unit tests.