Only display a diff when it is small enough

The diff display is rather slow now, probably because everything is
put into one big pre tag. Before we can fix this, we have to parse the
diff better. For now, just display a message.

html/commit.html

@@ -27,5 +27,5 @@
 	</table>
 	<hr>
 	<div id="message"></div>
-	<pre><div id="details"></div></pre>
+	<pre><code id="details"></code></pre>
 </body>