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>

sequencer.c

@@ -508,7 +508,8 @@ static int is_index_unchanged(void)
 		if (cache_tree_update(&the_index, 0))
 			return error(_("unable to update cache tree\n"));
 
-	return !hashcmp(active_cache_tree->sha1, head_commit->tree->object.oid.hash);
+	return !oidcmp(&active_cache_tree->oid,
+		       &head_commit->tree->object.oid);
 }
 
 static int write_author_script(const char *message)