Merge branch 'vd/sparse-stash'

Teach "git stash" to work better with sparse index entries.

* vd/sparse-stash:
  unpack-trees: preserve index sparsity
  stash: apply stash using 'merge_ort_nonrecursive()'
  read-cache: set sparsity when index is new
  sparse-index: expose 'is_sparse_index_allowed()'
  stash: integrate with sparse index
  stash: expand sparse-checkout compatibility testing