func (d *driver) scratchFilePrefix(file *pfs.File) (string, error) {
	return path.Join(d.scratchCommitPrefix(file.Commit), file.Path), nil
}
func (d *driver) scratchFilePrefix(file *pfs.File) (string, error) {
	return path.Join(d.scratchCommitPrefix(file.Commit), file.Path), nil
}
// scratchFilePrefix returns an etcd prefix that's used to temporarily
// store the state of a file in an open commit.  Once the commit is finished,
// the scratch space is removed.