diff: ignore --ita-[in]visible-in-index when diffing worktree-to-tree

diff: ignore --ita-[in]visible-in-index when diffing worktree-to-tree

This option is supposed to fix the diff of "diff-files" (not reporting
ita entries as new files) and "diff-index --cached <tree>" (showing ita
entries as present in the index with empty content) but not
"diff-index <tree>".

When --ita-invisible-in-index is set on "git diff-index <tree>",
unpack_trees() will eventually call oneway_diff() on the ita entry
with the same code flow as "diff-index --cached <tree>". We want to
ignore the ita entry for "diff-index --cached <tree>" but not
"diff-index <tree>" since the latter will examine and produce a diff
based on worktree entry's (real) content, not ita index entry's
(empty) content.

Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>