Don't use stdout of mergeMessageFileList.php

We recently had an incident where the output of
mergeMessageFileList.php was polluted by display_errors, causing the
resulting error text to be displayed for all requests to the relevant
wikis (the error was before the <?php).

In general, it's not so easy to ensure that stdout is clean, when you
are sharing code with web requests, and it's only a few extra lines of
shell script to use the pre-existing --output option.

Change-Id: I037a1f5e5fcea627f7f3ca75528bca18cdbfab1a