diff: rename struct diff_filespec's sha1_valid member

Now that this struct's sha1 member is called "oid", update the comment
and the sha1_valid member to be called "oid_valid" 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_valid
+ o.oid_valid

@@
struct diff_filespec *p;
@@
- p->sha1_valid
+ p->oid_valid

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

diffcore.h

@@ -33,7 +33,7 @@ struct diff_filespec {
 	int count;               /* Reference count */
 	int rename_used;         /* Count of rename users */
 	unsigned short mode;	 /* file mode */
-	unsigned sha1_valid : 1; /* if true, use sha1 and trust mode;
+	unsigned oid_valid : 1;  /* if true, use oid and trust mode;
 				  * if false, use the name and read from
 				  * the filesystem.
 				  */