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-break.c

@@ -57,7 +57,7 @@ static int should_break(struct diff_filespec *src,
 		return 1; /* even their types are different */
 	}
 
-	if (src->sha1_valid && dst->sha1_valid &&
+	if (src->oid_valid && dst->oid_valid &&
 	    !oidcmp(&src->oid, &dst->oid))
 		return 0; /* they are the same */