git-addons: comment out broken subtree command presence check
git-addons: comment out broken subtree command presence check

Broken by Git 2.20.0 or 2.20.1.

Thanks to mz4wheeler for the notice.
ilscipio/scipio-erp,ilscipio/scipio-erp,ilscipio/scipio-erp,ilscipio/scipio-erp,ilscipio/scipio-erp