Revert "Use /opt/git instead of /opt/stack/new"

This reverts commit 0092a3dd8cf98248278c1c3d0963ccf2d3f1ea00.

We need to revert this change, it's causing our CI jobs to not
actually test the submitted patches.

Change-Id: I3dd65e49c49959cdc91342e513591862036e7705
Merge "Revert "Use /opt/git instead of /opt/stack/new""