Allow git deploy from detached HEAD

[preaction]

It would be nice if we could quickly roll back to an old version of the site if it turns out there's a bug in the current version. One way to do this would be to checkout a specific commit SHA (or like HEAD~5), which would mean we aren't on a branch. Test to make sure this works correctly.

Like branches, if we have to deploy to another branch, we should go back to the place we came from.

If we can't make this work right, then being on a detached HEAD should probably give an error message