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