sparse-checkout: remove use of the_repository

sparse-checkout: remove use of the_repository

The logic for the 'git sparse-checkout' builtin uses the_repository all
over the place, despite some use of a repository struct in different
method parameters. Complete this removal of the_repository by using
'repo' when possible.

In one place, there was already a local variable 'r' that was set to
the_repository, so move that to a method parameter.

We cannot remove the USE_THE_REPOSITORY_VARIABLE declaration as we are
still using global constants for the state of the sparse-checkout.

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