read-cache.c: refactor set_new_index_sparsity() for subsequent commit

read-cache.c: refactor set_new_index_sparsity() for subsequent commit

Refactor code added to set_new_index_sparsity() in [1] to eliminate
indentation resulting from putting the body of his function within the
"if" block. Let's instead return early if we have no
istate->repo. This trivial change makes the subsequent commit's diff
smaller.

1. 491df5f679f (read-cache: set sparsity when index is new, 2022-05-10)

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Derrick Stolee <derrickstolee@github.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>