Merge branch 'ds/sparse-add'

"git sparse-checkout" learned a new "add" subcommand.

* ds/sparse-add:
  sparse-checkout: allow one-character directories in cone mode
  sparse-checkout: work with Windows paths
  sparse-checkout: create 'add' subcommand
  sparse-checkout: extract pattern update from 'set' subcommand
  sparse-checkout: extract add_patterns_from_input()

diff --cc Documentation/git-sparse-checkout.txt
Simple merge

diff --cc builtin/sparse-checkout.c
Simple merge

diff --cc dir.c
Simple merge