sparse-checkout: split out code for tweaking settings config
`init` has some code for handling updates to either cone mode or
the sparse-index setting. We would like to be able to reuse this
elsewhere, namely in `set` and `reapply`. Split this function out,
and make it slightly more general so it can handle being called from
the new callers.
Reviewed-by: Derrick Stolee <dstolee@microsoft.com>
Reviewed-by: Victoria Dye <vdye@github.com>
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>