[Build] Diff from branch point

Sometimes Jenkins posts [spurious reports of new classes being added](https://github.com/apache/spark/pull/2339#issuecomment-56570170). I believe this stems from diffing the patch against `master`, as opposed to against `master...`, which starts from the commit the PR was branched from.

This patch fixes that behavior.

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #2512 from nchammas/diff-only-commits-ahead and squashes the following commits:

c065599 [Nicholas Chammas] comment typo fix
a453c67 [Nicholas Chammas] diff from branch point