Add phutil_split_lines() for \r/\r\n/\n-aware line explosion

Summary:
We currently mishandle diffs with a mixture of \r, \n, and \r\n newlines (we discard them, but should retain them).

Add a function to correctly split a text corpus apart even if it has mixed newlines, since the regex wasn't exactly trivial.

Test Plan: Ran unit tests.

Reviewers: vrana, btrahan

Reviewed By: vrana

CC: aran

Maniphest Tasks: T866

Differential Revision: https://secure.phabricator.com/D3441