ui-snapshot: filter permitted snapshot requests

Currently the snapshots configuration option only filters which links
are displayed, not which snapshots may be generated and downloaded.
Apply the filter also to requests to ensure that the system policy is
enforced.

Signed-off-by: John Keeping <john@keeping.me.uk>
Reviewed-by: Christian Hesse <mail@eworm.de>

ui-snapshot.c

@@ -194,7 +194,7 @@ void cgit_print_snapshot(const char *head, const char *hex,
 	}
 
 	f = get_format(filename);
-	if (!f) {
+	if (!f || !(ctx.repo->snapshots & f->bit)) {
 		cgit_print_error_page(400, "Bad request",
 				"Unsupported snapshot format: %s", filename);
 		return;