Stop treating "\r" as a newline

Summary:
None of Git, Mercurial or SVN treat "\r" on its own as a newline under Unix/OS X or Windows. If you diff a file with \r newlines, you get a 1-line change. Do the same thing they do.

(Arguably we could split this into two functions like phutil_split_lines() and phutil_split_vcs_lines() or something, but I'm happy to call "\r" on its own a non-newline.)

Test Plan: Unit tests.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T1857

Differential Revision: https://secure.phabricator.com/D3757