rebuilding system messages in 1.4

I'm missing something obvious - how do I rebuild the system messages in 1.4?

In 1.3 I would call "php.exe rebuildmessages.php" which would do the job for
me.  But in 1.4 I get prompted to update or rebuild.  From the description
provided for these I'm not sure which one does what :-(

What do I need to call to simply refresh the messages based on our updated
Special:Allmessages ?

Cheers,

al.