merge-recursive: convert struct stage_data to use object_id

Convert the anonymous struct within struct stage_data to use struct
object_id.  The following Coccinelle semantic patch was used to
implement this, followed by the transformations in object_id.cocci:

@@
struct stage_data o;
expression E1;
@@
- o.stages[E1].sha
+ o.stages[E1].oid.hash

@@
struct stage_data *p;
expression E1;
@@
- p->stages[E1].sha
+ p->stages[E1].oid.hash

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