Fix t4031

When I tweaked the patch to use $SHELL_PATH instead of a hard-coded
"#!/bin/sh" to produce 3aa1f7c (diff: respect textconv in rewrite diffs,
2008-12-09), I screwed up.  This should fix it.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

t/t4031-diff-rewrite-binary.sh

@@ -46,7 +46,7 @@ test_expect_success 'rewrite diff can show binary patch' '
 
 {
 	echo "#!$SHELL_PATH"
-	cat >dump <<'EOF'
+	cat <<'EOF'
 perl -e '$/ = undef; $_ = <>; s/./ord($&)/ge; print $_' < "$1"
 EOF
 } >dump