sparse-checkout: use new update_sparsity() function

sparse-checkout: use new update_sparsity() function

Remove the equivalent of 'git read-tree -mu HEAD' in the sparse-checkout
codepaths for setting the SKIP_WORKTREE bits and instead use the new
update_sparsity() function.

Note that when an issue is hit, the error message splits 'error' and
'Cannot update sparse checkout' on separate lines.  For now, we use two
greps to find both pieces of the error message but subsequent commits
will clean up the messages reported to the user.

Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>