fix pagediff to not display as "preview"

I think originally, the page preview header was not displayed,
so diff was hacked in using it.

IkiWiki/Plugin/editdiff.pm

@@ -71,7 +71,7 @@ sub formbuilder_setup {
 		$content=~s/\r/\n/g;
 
 		my $diff = diff(srcfile($pagesources{$page}), $content);
-		$form->tmpl_param("page_preview", $diff);
+		$form->tmpl_param("page_diff", $diff);
 	}
 }
 

templates/editpage.tmpl

@@ -54,3 +54,12 @@
 <TMPL_VAR PAGE_PREVIEW>
 </div>
 </TMPL_IF>
+<TMPL_IF NAME="PAGE_DIFF">
+<hr />
+<div class="header">
+<span>Diff:</span>
+</div>
+<div id="diff">
+<TMPL_VAR PAGE_DIFF>
+</div>
+</TMPL_IF>