emacs: include tags from excluded messages in tag tab completion

The new message exclude functionality will hide tags that only exist
on excluded messages.  However, one might very well want to manually
modify excluded tags.  This makes sure tags from excluded messages are
always available in tab completion.

emacs/notmuch.el

@@ -89,7 +89,7 @@ For example:
    (with-output-to-string
      (with-current-buffer standard-output
        (apply 'call-process notmuch-command nil t
-	      nil "search" "--output=tags" search-terms)))
+	      nil "search" "--output=tags" "--exclude=false" search-terms)))
    "\n+" t))
 
 (defun notmuch-select-tag-with-completion (prompt &rest search-terms)