Manually manage mediawiki's checkout

We're managing mediawiki via git, but ensuring latest is a bad
idea, since we're using the wmf branches, and not master. If we
use ensure => latest, vcsrepo will continuously switch the branch
to master.

Change-Id: Ibb6f8758224798e0e90f81bbb64da8cb7dffa03a
Reviewed-on: https://review.openstack.org/20184
Reviewed-by: Jeremy Stanley <fungi@yuggoth.org>
Reviewed-by: Paul Belanger <paul.belanger@polybeacon.com>
Reviewed-by: James E. Blair <corvus@inaugust.com>
Reviewed-by: matthew-wagoner <matthew.wagoner@hp.com>
Approved: James E. Blair <corvus@inaugust.com>
Tested-by: Jenkins