Merge branch 'ds/sparse-index-ignored-files'

In cone mode, the sparse-index code path learned to remove ignored
files (like build artifacts) outside the sparse cone, allowing the
entire directory outside the sparse cone to be removed, which is
especially useful when the sparse patterns change.

* 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

diff --cc builtin/add.c
Simple merge

diff --cc dir.c
Simple merge

diff --cc dir.h
Simple merge

diff --cc sparse-index.c
Simple merge

diff --cc unpack-trees.c
Simple merge