index: make the index file format extensible.

... and move the cache-tree data into it.

Signed-off-by: Junio C Hamano <junkio@cox.net>
diff --git a/checkout-index.c b/checkout-index.c
index dd6a2d8..e56c354 100644
--- a/checkout-index.c
+++ b/checkout-index.c
@@ -39,6 +39,7 @@
 #include "cache.h"
 #include "strbuf.h"
 #include "quote.h"
+#include "cache-tree.h"
 
 #define CHECKOUT_ALL 4
 static const char *prefix;