sparse checkout UI

Hi,

I gave up on subtree checkout, the cvs/svn way (making subtree toplevel worktree). Instead I have been working on sparse checkout, which better fits Git's "working at toplevel worktree" philosophy. Things are shaping up, but I still have not figured out UI for entering/leaving/updating sparse checkout.

Should it be an option for git-checkout, like "git checkout --sparse=git-gui"? Or making it a separate command? git-clone may need "--sparse-checkout" option anyway, to be able to do sparse checkout from the beginning.

Opinions?