Construct diffs with attached changesets, even if empty

Summary: See discussion in IRC. Not 100% sure what's going on here because of email ghost theives, but conceivably a commit with no changes will end up with `null` changesets instead of `array()` changesets, which throws. Such diffs are certianly possible (`git commit --allow-empty`) even if they aren't the issue in this specific case. See T3416. Initialize changesets to `array()` to avoid throwing.

Test Plan:
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iiam

Reviewers: btrahan, chad

Reviewed By: chad

CC: aran

Differential Revision: https://secure.phabricator.com/D6339