apply: make i-t-a entries never match worktree

apply: make i-t-a entries never match worktree

By definition, an intent-to-add index entry can never match the
worktree, because worktrees have no concept of intent-to-add entries.
Therefore, "apply --index" should always fail on intent-to-add paths.

Because check_preimage() calls verify_index_match(), it already fails
for patches other than creation patches, which check_preimage() ignores.
This patch adds a check to check_preimage()'s rough equivalent for
creation patches, check_to_create().

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