t: refactor tests depending on Perl for textconv scripts

We have a couple of tests that depend on Perl for textconv scripts.
Refactor these tests to instead be implemented via shell utilities so
that we can drop a couple of PERL_TEST_HELPERS prerequisites.

Note that the conversion in t4030 is not a one-to-one equivalent to the
previous textconv script. Before this change we used to essentially do a
hexdump via Perl. The obvious conversion here would be to use `test-tool
hexdump` like we do for the other tests. But this would lead to a ripple
effect where we would have to adapt a bunch of other tests with a bunch
of seemingly unrelated changes, which would be somewhat awkward.

Instead, we're going with the minimum viable change: the test files we
write contain "\001" and "\000", and the test's expectation is that
those get translated into proper ASCII characters. So instead of doing a
full hexdump, we simply use tr(1) to translate these specific bytes.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4031-diff-rewrite-binary.sh

@@ -57,24 +57,19 @@ test_expect_success 'diff --stat counts binary rewrite as 0 lines' '
 	grep " rewrite file" diff
 '
 
-{
-	echo "#!$SHELL_PATH"
-	cat <<'EOF'
-"$PERL_PATH" -e '$/ = undef; $_ = <>; s/./ord($&)/ge; print $_' < "$1"
-EOF
-} >dump
-chmod +x dump
-
 test_expect_success 'setup textconv' '
+	write_script dump <<-\EOF &&
+	test-tool hexdump <"$1"
+	EOF
 	echo file diff=foo >.gitattributes &&
 	git config diff.foo.textconv "\"$(pwd)\""/dump
 '
 
-test_expect_success PERL_TEST_HELPERS 'rewrite diff respects textconv' '
+test_expect_success 'rewrite diff respects textconv' '
 	git diff -B >diff &&
-	grep "dissimilarity index" diff &&
-	grep "^-61" diff &&
-	grep "^-0" diff
+	test_grep "dissimilarity index" diff &&
+	test_grep "^-3d 0a 00" diff &&
+	test_grep "^+3d 0a 01" diff
 '
 
 test_done