t4023: fix ignored exit codes of git

t4023: fix ignored exit codes of git

Change a "git diff-tree" command to be &&-chained so that we won't
ignore its exit code, see the ea05fd5fbf7 (Merge branch
'ab/keep-git-exit-codes-in-tests', 2022-03-16) topic for prior art.

This fixes code added in b45563a229f (rename: Break filepairs with
different types., 2007-11-30). Due to hiding the exit code we hid a
memory leak under SANITIZE=leak.

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