R1:f6cb51562efd

Use phutil_split_lines() in Differential

Summary:
- We currently treat "\r" as a newline, but should not because VCSes do not.
- We get an extra empty line at the end of diffs created after D3442 because we now retain newlines.
- Historically we've converted tab pre-cache, but do it post-cache instead so we can add prefs about it, as we should handle it better than we do (e.g., let the user set it to a different width, infer width from comments in the file, expand it to actual tab stops, or show it…