Simplify DifferentialHunk::getAddedLines()

Summary: I will also need getRemovedLines() so refactor this first.

Test Plan:
New test case.
Viewed uncached diff.
Verified that the only callsite of getAddedLines() trims lines.

Reviewers: epriestley

Reviewed By: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D2875