diff-files --raw: show correct post-image of intent-to-add files

diff-files --raw: show correct post-image of intent-to-add files

The documented behavior of `git diff-files --raw` is to display

	[...] 0{40} if creation, unmerged or "look at work tree".

on the right hand (i.e. postimage) side. This happens for files that
have unstaged modifications, and for files that are unmodified but
stat-dirty.

For intent-to-add files, we used to show the empty blob's hash instead.
In c26022ea8f5 (diff: convert diff_addremove to struct object_id,
2017-05-30), we made that worse by inadvertently changing that to the
hash of the empty tree.

Let's make the behavior consistent with files that have unstaged
modifications (which applies to intent-to-add files, too) by showing
all-zero values also for intent-to-add files.

Accordingly, this patch adjusts the expectations set by the regression
test introduced in feea6946a5b (diff-files: treat "i-t-a" files as
"not-in-index", 2020-06-20).

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>