emacs: Fix '+' and '-' in case of thread no longer matching current search.

emacs: Fix '+' and '-' in case of thread no longer matching current search.

Similar to the way thread-viewing was broken after a thread was
archived, (and recently fixed), tag manipulation has also been broken
when the thread no longer matches the current search.

This also means that the behavior of '+' and '-' are now different
than that of '*'. The '+' and '-' bindings now return to the previous
behavior old affecting all messages in the thread, (and not simply
those matching the search).

I actually prefer this behavior, since otherwise a '-' operation on a
thread might not actually remove the tag from the thread, (since it
could operate on a subset of the thread and not hit all messages with
the given tag).

So I'd now like to fix '*' to be consistent with '+' and '-', for
which we add an item to TODO.