t6020: fix incompatible parameter expansion
Ævar reported that the function `make_user_friendly_and_stable_output()`
failed on a i386 box (gcc45) in the gcc farm boxes with error:
sed: couldn't re-allocate memory
It turns out that older versions of bash (4.3) or dash (0.5.7) cannot
evaluate expression like `${A%${A#???????}}` used to get the leading 7
characters of variable A.
Replace the incompatible parameter expansion so that t6020 works on
older version of bash or dash.
Reported-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>