t4140: test apply --intent-to-add interactions

t4140: test apply --intent-to-add interactions

Test that applying a new file creation patch with --intent-to-add to
an existing index does not modify the index outside adding the correct
intents-to-add, and that applying a patch with both modifications
and new file creations with --intent-to-add correctly only adds
intents-to-add to the index.

Signed-off-by: Raymond E. Pasco <ray@ameretat.dev>
Signed-off-by: Junio C Hamano <gitster@pobox.com>