Add `--force` option to `spack repo update`

[CodeGat]

Currently, if a repo has been force-pushed on the remote and is given spack repo update, it will fail to update. There is no --force option for spack repo update, and so one would have to run the equivalent git -C path/to/hashed/access-spack-packages reset --hard origin/api-v2, which makes the utility of spack managing our package repositories a lot less.

Consider adding a --force option to spack repo update that will force checkout a ref.