Merge branch 'ds/sparse-index-ignored-files' into sg/test-split-index-fix

* ds/sparse-index-ignored-files: sparse-checkout: clear tracked sparse dirs sparse-index: add SPARSE_INDEX_MEMORY_ONLY flag attr: be careful about sparse directories sparse-checkout: create helper methods sparse-index: use WRITE_TREE_MISSING_OK sparse-index: silently return when cache tree fails unpack-trees: fix nested sparse-dir search sparse-index: silently return when not using cone-mode patterns t7519: rewrite sparse index test