R1:6c1c3c3a7a7a

Fix line count in diffs with more hunks per changeset

Summary: This error usually doesn't occur because we have only one hunk per changeset most of the time.

Test Plan:
$hunk = new ArcanistDiffHunk();
$hunk->setAddLines(1);
$hunk->setDelLines(1);

$change = new ArcanistDiffChange();
$change->addHunk($hunk);
$change->addHunk($hunk);

$diff = DifferentialDiff::newFromRawChanges(array($change));
var_dump($diff->getLineCount());

Reviewers: epriestley

Reviewed By: epriestley

CC: aran…