(bug 12701) Use diff of all unseen revisions in the "new messages" bar.

(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.

Change-Id: I8870111802085d0bd188cb508c4f4b852985634d