wt-status: show sparse checkout status as well

wt-status: show sparse checkout status as well

Some of the early feedback of folks trying out sparse-checkouts at
$dayjob is that sparse checkouts can sometimes be disorienting; users
can forget that they had a sparse-checkout and then wonder where files
went.  Add some output to 'git status' in the form of a simple line that
states:

    You are in a sparse checkout with 35% of files present.

where, obviously, the exact figure changes depending on what percentage
of files from the index do not have the SKIP_WORKTREE bit set.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>