PullRequest: Fix IllegalStateException caused by be9a83.

Remove unnecessary @defining clause which causes the exception because
of calling PullRequest.getDiff.
PullRequest: Fix incorrect rendering of changes.

The fix of be9a838 was incorrect. PullRequest's diff should compare
merged commit and its parent, not its left parent and right parent.
Merge pull request #141 from npcode/yobi refs/heads/fix/pull-request-incorrect-diff-2