Convert struct cache_tree to use struct object_id

Convert the sha1 member of struct cache_tree to struct object_id by
changing the definition and applying the following semantic patch, plus
the standard object_id transforms:

@@
struct cache_tree E1;
@@
- E1.sha1
+ E1.oid.hash

@@
struct cache_tree *E1;
@@
- E1->sha1
+ E1->oid.hash

Fix up one reference to active_cache_tree which was not automatically
caught by Coccinelle.  These changes are prerequisites for converting
parse_object.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

cache-tree.h

@@ -1,6 +1,7 @@
 #ifndef CACHE_TREE_H
 #define CACHE_TREE_H
 
+#include "cache.h"
 #include "tree.h"
 #include "tree-walk.h"
 
@@ -15,7 +16,7 @@ struct cache_tree_sub {
 
 struct cache_tree {
 	int entry_count; /* negative means "invalid" */
-	unsigned char sha1[20];
+	struct object_id oid;
 	int subtree_nr;
 	int subtree_alloc;
 	struct cache_tree_sub **down;