sparse-index: add ensure_correct_sparsity function
The `ensure_correct_sparsity` function is intended to provide a means of
aligning the in-core index with the sparsity required by the repository
settings and other properties of the index. The function first checks
whether a sparse index is allowed (per repository & sparse checkout pattern
settings). If the sparse index may be used, the index is converted to
sparse; otherwise, it is explicitly expanded with `ensure_full_index`.
Helped-by: Junio C Hamano <gitster@pobox.com> Signed-off-by: Victoria Dye <vdye@github.com> Reviewed-by: Elijah Newren <newren@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>