diff: convert struct diff_filespec to struct object_id

Convert struct diff_filespec's sha1 member to use a struct object_id
called "oid" instead.  The following Coccinelle semantic patch was used
to implement this, followed by the transformations in object_id.cocci:

@@
struct diff_filespec o;
@@
- o.sha1
+ o.oid.hash

@@
struct diff_filespec *p;
@@
- p->sha1
+ p->oid.hash

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

wt-status.c

@@ -432,7 +432,8 @@ static void wt_status_collect_changed_cb(struct diff_queue_struct *q,
 			d->worktree_status = p->status;
 		d->dirty_submodule = p->two->dirty_submodule;
 		if (S_ISGITLINK(p->two->mode))
-			d->new_submodule_commits = !!hashcmp(p->one->sha1, p->two->sha1);
+			d->new_submodule_commits = !!oidcmp(&p->one->oid,
+							    &p->two->oid);
 	}
 }