sparse: rename command to debugsparse Sparse checkout is still highly experimental and not protected by BC guarantees yet. We also haven't had a discussion on the UX. To discourage use, we rename the sparse command to debugsparse.

sparse: remove reference to simplecache This is a 3rd party extension authored by Facebook. References in core are not appropriate. It will be possible to restore this code/optimization via monkeypatching. So Facebook won't lose any functionality. The removed code is important for performance. So add a comment tracking it.