Add all=1 query param for atom feeds

Displays all items from all branches in one feed

Signed-off-by: Aaron Griffin <agriffin@datalogics.com>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>

cgit.h

@@ -145,6 +145,7 @@ struct cgit_query {
 	char *sort;
 	int showmsg;
 	int ssdiff;
+	int show_all;
 };
 
 struct cgit_config {