diff --no-index tests: test mode normalization

diff --no-index tests: test mode normalization

When "git diff --no-index X Y" is run the modes of the files being
differ are normalized by canon_mode() in fill_filespec().

I recently broke that behavior in a patch of mine[1] which would pass
all tests, or not, depending on the umask of the git.git checkout.

Let's test for this explicitly. Arguably this should not be the
behavior of "git diff --no-index". We aren't diffing our own objects
or the index, so it might be useful to show mode differences between
files.

On the other hand diff(1) does not do that, and it would be needlessly
distracting when e.g. diffing an extracted tar archive whose contents
is the same, but whose file modes are different.

1. https://lore.kernel.org/git/20210316155829.31242-2-avarab@gmail.com/

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>