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>

cgit.h

@@ -143,6 +143,7 @@ struct cgit_query {
 	int nohead;
 	char *sort;
 	int showmsg;
+	int ssdiff;
 };
 
 struct cgit_config {
@@ -194,6 +195,7 @@ struct cgit_config {
 	int summary_branches;
 	int summary_log;
 	int summary_tags;
+	int ssdiff;
 	struct string_list mimetypes;
 	struct cgit_filter *about_filter;
 	struct cgit_filter *commit_filter;