Merge branch 'rp/apply-intent-to-add-fix'

"git apply -N" should start from the current index and register
only new files, but it instead started from an empty index, which
has been corrected.

* rp/apply-intent-to-add-fix:
  apply docs: clarify wording for --intent-to-add
  t4140: test apply --intent-to-add interactions
  apply: only write intents to add for new files
  apply: read in the index in --intent-to-add mode