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 visually in some way).

Test Plan:

• Verified diffs no longer have an empty line at the end.
• Created a diff of a "\r" file and verified it displayed somewhat reasonably. All browsers treat "\r" as a real newline so it's not necessarily perfect, but we can clean that up later. Hopefully these files are exceedingly rare.
• Created a file with tabs and verified it came out reasonably.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1857

Differential Revision: https://secure.phabricator.com/D3759