Fix diff mode switching when side-by-side-diffs=1

When side-by-side-diffs=1 was set in cgitrc, specyfing 'ss=0' in the query-
string would not switch to unified diffs. This patch fixes the issue by
introducing a separate variable to track the occurrence of "ss" in the
querystring.

cgit.h

@@ -133,6 +133,7 @@ struct reflist {
 struct cgit_query {
 	int has_symref;
 	int has_sha1;
+	int has_ssdiff;
 	char *raw;
 	char *repo;
 	char *page;