DiffHilighter: Specific the div in which to hilight

Previously, the diff hilighter was a bit odd in that it would
just search for elements with a diff and highlight those. While
useful in theory, this wasn't used at all. We change it to
receive the diff to hilight, which makes the code somewhat simpler.

html/css/diff.css

@@ -11,20 +11,20 @@ body {
 	border-top:0.1em solid #999999;
 }
 
-code .delline, code .oldfile {
+.delline, .oldfile {
   background-color: #FEE;
   color: #B00;
 }
 
-code .addline, code .newfile {
+.addline, .newfile {
   background-color: #DFD;
   color: #080;
 }
-code .fileline {
+.fileline {
   font-weight: bold;
 }
 
-code .hunkheader {
+.hunkheader {
   background-color: #f7f7f7;
   color: #bbb;
 }