t4129: test that git apply warns for unexpected mode changes

t4129: test that git apply warns for unexpected mode changes

There is no test covering what commit 01aff0a (apply: correctly reverse
patch's pre- and post-image mode bits, 2023-12-26) addressed. Prior to
that commit, git apply was erroneously unaware of a file's expected mode
while reverse-patching a file whose mode was not changing.

Add the missing test coverage to assure that git apply is aware of the
expected mode of a file being patched when the patch does not indicate
that the file's mode is changing. This is achieved by arranging a file
mode so that it doesn't agree with patch being applied, and checking git
apply's output for the warning it's supposed to raise in this situation.
Test in both reverse and normal (forward) directions.

Signed-off-by: Mark Mentovai <mark@chromium.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>