Revert "Merge branch '2065-remove-deprecated-git-data-dir' into 'master'"

This reverts commit 1c75edce, reversing changes made to a6dbd2b7.

See https://gitlab.com/gitlab-org/omnibus-gitlab/issues/2757 for discussion.

But essentially the change we are reverting will probably cause downtime for some users. For now we will revert it. I will try to get in a better change for 10.0, but we may keep the support for git_data_dir until 11.0 if it doesn't make it in.

cc\ @stanhu @joshlambert