Fix various newline problems in the difference engines

Summary: I'll mark this one up inline since it's all separate bugs.

Test Plan:

• Created a diff with eight changes: (newline absent -> newline present, newline present -> newline absent, newline present -> newline present, newline absent -> newline absent) x (short file with change near end, long file with change near middle).
• Viewed diff in Ignore All, Ignore Most, Ignore Trailing and Show All whitespace modes.
• All 32 results seemed sensible.
• Really wish this stuff was better factored and testable. Need to fix it. :(

Reviewers: vrana, btrahan

Reviewed By: btrahan

CC: aran, epriestley

Maniphest Tasks: T1030

Differential Revision: https://secure.phabricator.com/D1992