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