HTML: Improve diff display performance

This improves the diff performance by a factor of around 10. Things done
to do this are, among others:

	* user String.replace(//g) instead of String.gsub(). It's
		*much* faster
	* Don't use anonymous inner functions for iteration, but instead
		use for loops
	* Don't use String.startsWith() if we can just compare a
		single character

Because we are now so much faster, we can also show bigger diffs
by default.

html/commit.js

@@ -139,7 +139,7 @@ var loadCommit = function() {
 
 	$("message").innerHTML = commit.message.replace(/\n/g,"<br>");
 
-	if (commit.diff.length < 10000) {
+	if (commit.diff.length < 200000) {
 		showDiffs();
 	} else {
 		$("details").innerHTML = "<a class='showdiff' href='' onclick='showDiffs(); return false;'>This is a large commit. Click here or press 'v' to view.</a>";