get-mw-core now fallback on git.wikimedia.org

On labs we do not have a local replication of mediawiki/core, instead
fallback to git.wikimedia.org.

The shell script is provided by https://gerrit.wikimedia.org/r/#/c/97513/

Change-Id: Ibd65854909e61b28d2689bd9d48b18ae6c0ba0d2