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?
-- 
Duy