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.c

@@ -281,6 +281,7 @@ static void querystring_cb(const char *name, const char *value)
 		ctx.qry.period = xstrdup(value);
 	} else if (!strcmp(name, "ss")) {
 		ctx.qry.ssdiff = atoi(value);
+		ctx.qry.has_ssdiff = 1;
 	} else if (!strcmp(name, "all")) {
 		ctx.qry.show_all = atoi(value);
 	} else if (!strcmp(name, "context")) {