sparse-checkout: split out code for tweaking settings config

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 <redacted>
Reviewed-by: Victoria Dye <redacted>
Signed-off-by: Elijah Newren <redacted>
Signed-off-by: Junio C Hamano <redacted>