(bug 12701) Use diff of all unseen revisions in the "new messages" bar.
Also pluralize properly.
"You have a new message from another user (last change)"
if only one unseen revision, or
"You have new messages [from another user|from N users|]
(last changes)" if there are several unseen revisions.
Contains a fix in Title::countAuthorsBetween() adding options to include
(one or both of) the delimiting revisions in the count.