Merge branch 'vd/sparse-read-tree'

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