Merge branch 'en/sparse-checkout'

Merge branch 'en/sparse-checkout'

"sparse-checkout" UI improvements.

* en/sparse-checkout:
  sparse-checkout: provide a new reapply subcommand
  unpack-trees: failure to set SKIP_WORKTREE bits always just a warning
  unpack-trees: provide warnings on sparse updates for unmerged paths too
  unpack-trees: make sparse path messages sound like warnings
  unpack-trees: split display_error_msgs() into two
  unpack-trees: rename ERROR_* fields meant for warnings to WARNING_*
  unpack-trees: move ERROR_WOULD_LOSE_SUBMODULE earlier
  sparse-checkout: use improved unpack_trees porcelain messages
  sparse-checkout: use new update_sparsity() function
  unpack-trees: add a new update_sparsity() function
  unpack-trees: pull sparse-checkout pattern reading into a new function
  unpack-trees: do not mark a dirty path with SKIP_WORKTREE
  unpack-trees: allow check_updates() to work on a different index
  t1091: make some tests a little more defensive against failures
  unpack-trees: simplify pattern_list freeing
  unpack-trees: simplify verify_absent_sparse()
  unpack-trees: remove unused error type
  unpack-trees: fix minor typo in comment