highlight: fix to work with caching templater

hgext/highlight/highlight.py

@@ -23,7 +23,7 b' SYNTAX_CSS = (\'\\n<link rel="stylesheet" '
 def pygmentize(field, fctx, style, tmpl):
 
     # append a <link ...> to the syntax highlighting css
-    old_header = ''.join(tmpl('header'))
+    old_header = tmpl.load('header')
     if SYNTAX_CSS not in old_header:
         new_header =  old_header + SYNTAX_CSS
         tmpl.cache['header'] = new_header