Fix t4030-diff-textconv.sh

Avoid passing cygwin pathnames to Perl. Some Perls have problems using them
Signed-off-by: NAlex Riesen <raa.lkml@gmail.com>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t4030-diff-textconv.sh

@@ -21,7 +21,7 @@ EOF
 
 cat >hexdump <<'EOF'
 #!/bin/sh
-perl -e '$/ = undef; $_ = <>; s/./ord($&)/ge; print $_' "$1"
+perl -e '$/ = undef; $_ = <>; s/./ord($&)/ge; print $_' < "$1"
 EOF
 chmod +x hexdump