statinfo: move stat_{data,validity} functions from cache/read-cache

These functions do not depend upon struct cache_entry or struct
index_state in any way, and it seems more logical to break them out into
this file, especially since statinfo.h already has the struct stat_data
declaration.

Diff best viewed with `--color-moved`.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

object.c

@@ -6,6 +6,7 @@
 #include "object-file.h"
 #include "object-store.h"
 #include "blob.h"
+#include "statinfo.h"
 #include "tree.h"
 #include "commit.h"
 #include "tag.h"