t6011: fix misconversion from perl to sed

No, this is not about a quiz on regexp compatibility between Perl
and sed.

Back when cdbdc6bf (t: refactor tests depending on Perl substitution
operator, 2025-04-03) rewrote many uses of perl with sed, the general
pattern of the original scripts were

    chmod +w some_read_only_file &&
    perl -p -e "regexp to munge" some_read_only_file >some_tmp &&
    mv some_tmp some_read_only_file

persumably because the author knew that replacing some_read_only_file
with "mv" at the last step would not work without "mv -f" in some
environments (GNU seems to succeed without giving any prompt when
not running interactively, which is what happens when running t/
scripts).  Replacing perl with sed would be fine as long as sed with
updated regexp does the equivalent munging.

But one place used to use a different construct in the original:

    perl -i.bak -p -e "regexp to munge" some_read_only_file

With _no_ temporary file or "mv", "perl -i" allows you to replace a
read-only file in place.

When we replaced the use of "perl" with "sed" in the said commit,
however, because "sed -i" is not portable, we rewrote that in-place
replacement to

    sed "regexp to munge" some_read_only_file >some_tmp &&
    mv some_tmp some_read_only_file

Again, unfortunately that does not work in some environment, without
"mv -f".

We could run "mv -f" here, but we would then need to remove "chmod
+w" and have them use "mv -f" instead at all places that were
touched cdbdc6bf (t: refactor tests depending on Perl substitution
operator, 2025-04-03) to be consistent (and more concise).

For now, let's make it consistent in the other direction by mimick
the other places that made the target read-write before moving.

Speaking of portability, the outcome of using "sed" on non-text
files is unspecified, so the entire exercise of cdbdc6bf may have
needed to be reverted if people still used ancient version of
"standard compliant" sed that barfs on non-text files, but these
days we may be able to get away with "BSDs and GNU seem OK with it"
;-)  But one fix at a time.

Reported-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t6011-rev-list-with-bad-commit.sh

@@ -38,6 +38,7 @@ test_expect_success 'verify number of revisions' \
 test_expect_success 'corrupt second commit object' '
 	for p in .git/objects/pack/*.pack
 	do
+		chmod +w "$p" &&
 		sed "s/second commit/socond commit/" "$p" >"$p.munged" &&
 		mv "$p.munged" "$p" ||
 		return 1