Occasionally, the only change to a review request will be to add additional

reviewers. This is especially common when a review request is first sent to a

generic group, and then people in that group make suggestions about who might

be more appropriate to review the code in question.
This change makes it so that when the only change to a review request is to

change the list of reviewers (people or groups), the e-mail that is sent is

sent only to the author of the review request (assuming that they have "own

actions" e-mails turned on), people who have starred the review request, and

the newly-added reviewers. This was requested by a user who has some very

high-volume e-mail lists, in an effort to cut down on the amount of traffic.
In the case where the change was to remove reviewers, the e-mail is sent only

to the author. In the case that the author has "own actions" e-mails turned

off, no e-mail will be sent.

Ran unit tests.