Telepathy-glib has a enum value for the MembersChanged signal to signify that a
user's ID has changed.  Previously, empathy was simply interpreting this as if a
user with the old name had left the chat and a different user with the new name
had entered the chat.  This change handles this case more gracefully by updating
the contact's id (and name) when this change reason is present

One thing that does not yet work with this patch is if you are engaged in a
private chat with a person and they change their nick in the middle of the chat.
Then the EmpathyContact* that you are chatting with is no longer the
EmpathyContact* representing the remote user, so messages won't be delivered
properly.  When we detect that a user has been 'renamed', we probably need to
somehow go through all of the private chats with that person and swap out the
old (invalid) EmpathyContact* and replace it with the new one so that the chat
can continue without interruption.