Merge branch 'vd/sparse-read-tree'

"git read-tree" has been made to be aware of the sparse-index feature. * vd/sparse-read-tree: read-tree: make three-way merge sparse-aware read-tree: make two-way merge sparse-aware read-tree: narrow scope of index expansion for '--prefix' read-tree: integrate with sparse index read-tree: expand sparse checkout test coverage read-tree: explicitly disallow prefixes with a leading '/' status: fix nested sparse directory diff in sparse index sparse-index: prevent repo root from becoming sparse