Add possibility to switch between unidiff and side-by-side-diff.

A new config option side-by-side-diffs added, defaulting to 0,
meaning unidiff. Also a query option (ss) is used toggle this.

In the commit page you can switch between the two diff formats by
clicking on the link on the "commit"-row, to the right of (patch).

In the diff page you can switch by using the link at the start
of the page.

All commit-links and diff-links will remember the choice.

Signed-off-by: Ragnar Ouchterlony <ragnar@lysator.liu.se>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

ui-refs.c

@@ -74,7 +74,7 @@ static int print_branch(struct refinfo *ref)
 	html("</td><td>");
 
 	if (ref->object->type == OBJ_COMMIT) {
-		cgit_commit_link(info->subject, NULL, NULL, name, NULL);
+		cgit_commit_link(info->subject, NULL, NULL, name, NULL, 0);
 		html("</td><td>");
 		html_txt(info->author);
 		html("</td><td colspan='2'>");