Merge branch 'ds/sparse-sparse-checkout'

Merge branch 'ds/sparse-sparse-checkout'

"sparse-checkout" learns to work well with the sparse-index
feature.

* ds/sparse-sparse-checkout:
  sparse-checkout: integrate with sparse index
  p2000: add test for 'git sparse-checkout [add|set]'
  sparse-index: complete partial expansion
  sparse-index: partially expand directories
  sparse-checkout: --no-sparse-index needs a full index
  cache-tree: implement cache_tree_find_path()
  sparse-index: introduce partially-sparse indexes
  sparse-index: create expand_index()
  t1092: stress test 'git sparse-checkout set'
  t1092: refactor 'sparse-index contents' test