Merge branch 'ds/sparse-cone'

Merge branch 'ds/sparse-cone'

Management of sparsely checked-out working tree has gained a
dedicated "sparse-checkout" command.

* ds/sparse-cone: (21 commits)
  sparse-checkout: improve OS ls compatibility
  sparse-checkout: respect core.ignoreCase in cone mode
  sparse-checkout: check for dirty status
  sparse-checkout: update working directory in-process for 'init'
  sparse-checkout: cone mode should not interact with .gitignore
  sparse-checkout: write using lockfile
  sparse-checkout: use in-process update for disable subcommand
  sparse-checkout: update working directory in-process
  sparse-checkout: sanitize for nested folders
  unpack-trees: add progress to clear_ce_flags()
  unpack-trees: hash less in cone mode
  sparse-checkout: init and set in cone mode
  sparse-checkout: use hashmaps for cone patterns
  sparse-checkout: add 'cone' mode
  trace2: add region in clear_ce_flags
  sparse-checkout: create 'disable' subcommand
  sparse-checkout: add '--stdin' option to set subcommand
  sparse-checkout: 'set' subcommand
  clone: add --sparse mode
  sparse-checkout: create 'init' subcommand
  ...