sparse-index: refactor path_found()

sparse-index: refactor path_found()

In advance of changing the behavior of path_found(), take all of the
intermediate data values and group them into a single struct. This
simplifies the method prototype as well as the initialization. Future
changes can be made directly to the struct and method without changing
the callers with this approach.

Note that the clear_path_found_data() method is currently empty, as
there is nothing to free. This method is a placeholder for future
changes that require a non-trivial implementation. Its stub is created
now so consumers could call it now and not change in future changes.

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