emacs: Avoid removing the unread tag due to internal navigation

emacs: Avoid removing the unread tag due to internal navigation

Sometimes the internals of the implementation navigate among messages,
(as opposed to the user explicitly requesting the next message to be
shown). In these cases we don't want to remove the unread tag from the
message navigated to.

This fixes a bug where invocation of notmuch-show-next-unread-message
would clear the unread tag from all messages in a thread.