Strip everything before the first pf token

In this isolate case,
{{safe<includeonly />subst:#if:{{{1}}}}}
only "subst:" is part of the same token as the prefix so
`normalizeTarget` doesn't take care of remove the "safesubst:"

The fix here is from noting that we wouldn't be at this place in the
code if the target didn't normalize a canonical function name, so the
contents before the prefix can be safely discarded.

This is another patch along the line of 5608e83 and bcd5572

Bug: T185643
Change-Id: I7002804fcf5305ffb6c88d93c201d9e972f9037f

lib/wt2html/tt/TemplateHandler.js

@@ -473,7 +473,7 @@ TemplateHandler.prototype.resolveTemplateTarget = function(state, targetToks) {
 	if (canonicalFunctionName !== undefined) {
 		// Extract toks that make up pfArg
 		var pfArgToks;
-		var re = new RegExp(prefix, 'i');
+		var re = new RegExp('^(.*?)' + prefix, 'i');
 
 		// Because of the lenient stringifying above, we need to find the prefix
 		var i = targetToks.findIndex(function(t) {