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.
It probably doesn't make sense to respect Git's `core.excludesFile`
config when not running in a Git-backed repo, but we also already
respect `.gitignore` files in the working copy regardless of backend,
so at least it's consistent with that. We can revisit it when the
native backend becomes a reasonable choice.
Closes#87.
