notmuch.el: Don't require an extra press of space bar before archiving.

I had implemented this intentionally originally, thinking that it
would be important to see the last message scroll all the way off
screen before the next press of the magic space bar would go and
archive away the whole thread.

But in practice, that just turns out to be annoying, (especially for a
long sequence of single-message threads where the space bar has to be
pressed twice for every one). It's actually quite easy to know if it's
"safe" to press the space bar expecting just a scroll instead of an
archive by simply looking down and seeing if the current window is
full.

And as for the total lack of undo with all of this, I'm getting by by
simply using x to get back to the search view, and then going back
into the thread of interest.

notmuch.el

@@ -358,11 +358,10 @@ which this thread was originally shown."
 	(unread (notmuch-show-message-unread-p)))
     (if (> next (window-end))
 	(scroll-up nil)
-      (if unread
+      (let ((last (notmuch-show-last-message-p)))
 	(notmuch-show-mark-read-then-next-open-message)
-	(if (notmuch-show-last-message-p)
-	    (notmuch-show-archive-thread)
-	  (notmuch-show-next-open-message))))))
+	(if last
+	    (notmuch-show-archive-thread))))))
 
 (defun notmuch-show-markup-citations-region (beg end)
   (goto-char beg)