This adds a `jj sparse` command with options to list and manage the
set of paths to include in the working copy. It only supports includes
(postive matches) for now.
I'm not sure "sparse" is the best name for the feature. Perhaps it
would make sense as a subcommand under `jj workspace` - maybe `jj
workspace track`? However, there's also `jj untrack` for removing a
file from the working copy and leaving it in the working copy. I'm
happy to hear suggestions, or we can get back to the naming later.
