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>

diffcore.h

@@ -25,7 +25,7 @@
 struct userdiff_driver;
 
 struct diff_filespec {
-	unsigned char sha1[20];
+	struct object_id oid;
 	char *path;
 	void *data;
 	void *cnt_data;