Convert struct object to object_id

struct object is one of the major data structures dealing with object
IDs.  Convert it to use struct object_id instead of an unsigned char
array.  Convert get_object_hash to refer to the new member as well.

Signed-off-by: brian m. carlson <sandals@crustytoothpaste.net>
Signed-off-by: Jeff King <peff@peff.net>
diff --git a/builtin/merge-base.c b/builtin/merge-base.c
index 08a8217..a891162 100644
--- a/builtin/merge-base.c
+++ b/builtin/merge-base.c
@@ -16,7 +16,7 @@
 		return 1;
 
 	while (result) {
-		printf("%s\n", sha1_to_hex(result->item->object.sha1));
+		printf("%s\n", oid_to_hex(&result->item->object.oid));
 		if (!show_all)
 			return 0;
 		result = result->next;
@@ -62,7 +62,7 @@
 		return 1;
 
 	while (result) {
-		printf("%s\n", sha1_to_hex(result->item->object.sha1));
+		printf("%s\n", oid_to_hex(&result->item->object.oid));
 		result = result->next;
 	}
 	return 0;
@@ -83,7 +83,7 @@
 		return 1;
 
 	while (result) {
-		printf("%s\n", sha1_to_hex(result->item->object.sha1));
+		printf("%s\n", oid_to_hex(&result->item->object.oid));
 		if (!show_all)
 			return 0;
 		result = result->next;
@@ -196,7 +196,7 @@
 		goto cleanup_return;
 	}
 
-	printf("%s\n", sha1_to_hex(bases->item->object.sha1));
+	printf("%s\n", oid_to_hex(&bases->item->object.oid));
 
 cleanup_return:
 	free_commit_list(bases);