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>

merge-recursive.c

@@ -304,7 +304,7 @@ struct tree *write_tree_from_memory(struct merge_options *o)
 		return NULL;
 	}
 
-	result = lookup_tree(active_cache_tree->sha1);
+	result = lookup_tree(active_cache_tree->oid.hash);
 
 	return result;
 }