Use 'forward-line' instead of 'next-line' while walking search display

The documentation for 'next-line' suggests that 'forward-line' is a
better choice for non-interactive usage. That appears to be the case
here; using next-line caused emacs to spin forever for me.

Signed-off-by: Keith Packard <keithp@keithp.com>

notmuch.el

@@ -791,7 +791,7 @@ global search.
     (goto-char (point-min))
     (while (not (eobp))
       (notmuch-search-markup-this-thread-id)
-      (next-line))))
+      (forward-line))))
 
 (defun notmuch-search-show-thread ()
   (interactive)