The new DVCS support has a different method for uploading diffs, and the

new support didn't trigger the auto-adding of default reviewers, meaning

that functionality was lost.
This change restores that by adding default reviewers when a commit

series is finalized. To prevent duplicate logic, it also introduces

ReviewRequest.can_add_default_reviewers(), which will check to make

sure that default reviewers are both supported for the review request

and that the no existing diffs have been published.

Unit tests pass.
Manually posted a change using a commit series that previously wasn't

applying default reviewers. Saw that the default reviewers were being

applied successfully.