Use newly added InvalidCommandId instead of 0

The symbol was added by 71901ab6d; the original code was introduced by
6868ed749.  Development of both overlapped which is why we apparently
failed to notice.

This is a (very slight) behavior change, so I'm not backpatching this to
9.4 for now, even though the symbol does exist there.

src/backend/access/heap/heapam.c

@@ -2818,7 +2818,7 @@ l1:
 		if (result == HeapTupleSelfUpdated)
 			hufd->cmax = HeapTupleHeaderGetCmax(tp.t_data);
 		else
-			hufd->cmax = 0;		/* for lack of an InvalidCommandId value */
+			hufd->cmax = InvalidCommandId;
 		UnlockReleaseBuffer(buffer);
 		if (have_tuple_lock)
 			UnlockTupleTuplock(relation, &(tp.t_self), LockTupleExclusive);
@@ -3415,7 +3415,7 @@ l2:
 		if (result == HeapTupleSelfUpdated)
 			hufd->cmax = HeapTupleHeaderGetCmax(oldtup.t_data);
 		else
-			hufd->cmax = 0;		/* for lack of an InvalidCommandId value */
+			hufd->cmax = InvalidCommandId;
 		UnlockReleaseBuffer(buffer);
 		if (have_tuple_lock)
 			UnlockTupleTuplock(relation, &(oldtup.t_self), *lockmode);
@@ -4573,7 +4573,7 @@ failed:
 		if (result == HeapTupleSelfUpdated)
 			hufd->cmax = HeapTupleHeaderGetCmax(tuple->t_data);
 		else
-			hufd->cmax = 0;		/* for lack of an InvalidCommandId value */
+			hufd->cmax = InvalidCommandId;
 		LockBuffer(*buffer, BUFFER_LOCK_UNLOCK);
 		if (have_tuple_lock)
 			UnlockTupleTuplock(relation, tid, mode);