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>

sparse-index.h

@@ -4,6 +4,7 @@
 struct index_state;
 #define SPARSE_INDEX_MEMORY_ONLY (1 << 0)
 int convert_to_sparse(struct index_state *istate, int flags);
+void ensure_correct_sparsity(struct index_state *istate);
 
 /*
  * Some places in the codebase expect to search for a specific path.