Sparse checkout in worktree

[Michael J Gruber <git@drmicha.warpmail.net>]

Hi there,

I'm wondering how much it would take to enable worktree specific sparse
checkouts. From a superfluous look:

- $GIT_DIR/info/sparse_checkout needs to be worktree specific
- We don't have much tooling around sparse to speak of at all.

The endgoal would be to have something like

git checkout [--sparse <pattern>]...

which sets up the sparse_checkout file and "git worktree" to pass any
--sparse option on to "git checkout".

While in an ideal world we all have micro repos, in the real world we
often have larger repos with mostly independent subdirs. For a quick fix
on a side branch in a subdir, a new sparse worktree would be an ideal
lean solution.

As it is, "git stash save && git checkout" is leaner but interrupts the
workflow more, and a local "git clone" with links and alternates is
leaner, too, but conceptually overkill if you want to work quickly on an
existing side branch.

Cheers,
Michael