message: Updates uberenv to use 'git checkout' rather than 'git reset --hard' for its `spack_commit`
  The new way puts our local spack repo in a detached head state rather than making the
  branch pointer for spack develop branch point to our arbitrary commit.
  See:
diff: @@ -428,7 +428,7 @@ def main():
  sha1 = project_opts["spack_commit"]
  print "[info: using spack commit %s]" % sha1
  os.chdir(pjoin(dest_dir,"spack"))
  - sexe("git reset --hard %s" % sha1,echo=True)
  + sexe("git checkout %s" % sha1,echo=True)
  if opts["spack_pull"]:
  # do a pull to make sure we have the latest
  