Merge branch 'nd/diff-apply-ita'

Merge branch 'nd/diff-apply-ita'

"git diff" compares the index and the working tree.  For paths
added with intent-to-add bit, the command shows the full contents
of them as added, but the paths themselves were not marked as new
files.  They are now shown as new by default.

"git apply" learned the "--intent-to-add" option so that an
otherwise working-tree-only application of a patch will add new
paths to the index marked with the "intent-to-add" bit.

* nd/diff-apply-ita:
  apply: add --intent-to-add
  t2203: add a test about "diff HEAD" case
  diff: turn --ita-invisible-in-index on by default
  diff: ignore --ita-[in]visible-in-index when diffing worktree-to-tree