How to deploy code/Core submodule update

While Gerrit performs automatic MW core submodule updates for most extensions when you merge a commit to a wmf/ branch in them, it's confused by VisualEditor core's repo (VisualEditor/VisualEditor) and doesn't do it for the VE extension.

Additionally, our deployment branches are not set up to create automatic submodule updates for extensions using custom deployment branches. This process applies to those as well as VisualEditor.

Here's how to do it manually, starting from MW core:

git fetch
git branch --track wmf/1.20wmf1 origin/wmf/1.20wmf1
git checkout wmf/1.20wmf1
git pull
git submodule update --init --recursive extensions/VisualEditor
cd extensions/VisualEditor
git fetch
git checkout origin/wmf/1.20wmf1
cd ../..
git diff extensions/VisualEditor
git commit -m "Update VisualEditor for I{CHANGE_ID_HERE}" extensions/VisualEditor
git review