Upgrading Mediawiki through Git seemed like a cool idea and worked quite well for a long time. But since Mediawiki 1.25 the update process changed considerably and just wasn't fun any more. As updates are a rare occurence anyway, I decided to switch back to tarballs instead. Let's try this, for Mediawiki 1.27:

 curl https://www.mediawiki.org/keys/keys.txt | gpg --import
 wget https://releases.wikimedia.org/mediawiki/1.27/mediawiki-1.27.1.tar.gz{,.sig}
 gpg --verify mediawiki-1.27.1.tar.gz.sig
 
 export DOCROOT=/var/www/
 cd $DOCROOT/mediawiki
 tar --strip-components=1 -xzf ~/mediawiki-1.27.1.tar.gz

Perform the necessary (database) updates:

 cd $DOCROOT/mediawiki
 script -a -c "date; php maintenance/update.php --conf `pwd`/LocalSettings.php" ~/mwupdate.log

While we're at it, re-generate the sitemap:

 cd $DOCROOT/mediawiki
 mkdir -p sitemap && chmod 0770 sitemap && sudo chgrp www-data sitemap
 sudo -u www-data MW_INSTALL_PATH=`pwd` php maintenance/generateSitemap.php \
     --conf `pwd`/LocalSettings.php --fspath `pwd`/sitemap --server https://www.example.net \
     --urlpath https://www.example.net/mediawiki/sitemap --skip-redirects

Remove/disable clutter:

 cd $DOCROOT/mediawiki
 rm -rf COPYING CREDITS FAQ HISTORY INSTALL README RELEASE-NOTES-1.27 UPGRADE
 chmod 0 docs maintenance tests
 sudo touch {cache,images}/index.html

Don't forget to upgrade the extensions as well:

 cd ../piwik-mediawiki-extension-git
 git checkout master && git pull && git clean -dfx
 git archive --prefix=piwik-mediawiki-extension/ --format=tar HEAD | tar -C $DOCROOT/mediawiki/extensions/ -xvf -
  
 cd ../MobileFrontend-git
 git checkout master && git pull && git clean -dfx
 git archive --prefix=MobileFrontend/ --format=tar origin/REL1_27  | tar -C $DOCROOT/mediawiki/extensions/ -xvf -

And with that, the new version should be online :-)