I am happy to report, that the merge has been completed on the contrib branch! There is a slight trick to get your local clone to work, if you merge in contrib, because Git might complain that you already have a Umbraco.Web.UI.Client folder. Simply delete the existing folder before merging contrib in to your own branches and it should work:

bash
rm -rf src/Umbraco.Web.UI.Client
git merge upstream/contrib