notmuch.el: Don't use beginning-of-buffer from elisp program.

This silences a warning when compiling notmuch.el. The documentation
of beginning-of-buffer does say (rather emphatically) that it's not
to be used from lisp programs.

notmuch.el

@@ -877,7 +877,7 @@ thread from that buffer can be show when done with this one)."
                           (setq btn (forward-button 1))
                         (error (setq btn nil)))
                     ))
-                  (beginning-of-buffer)
+                  (goto-char (point-min))
                   ))))
       )))