t0021: use $SHELL_PATH for the filter script

On Windows, we need the shbang line to correctly invoke shell scripts via
a POSIX shell, except when the script is invoked via 'sh -c' because sh (a
bash) does "the right thing".  But the clean and smudge filters will not
always be invoked via 'sh -c'; to futureproof, we should mark the the one
in t0021-conversion with #!$SHELL_PATH.
Signed-off-by: NJohannes Sixt <j6t@kdbg.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

t/t0021-conversion.sh

@@ -4,7 +4,8 @@ test_description='blob conversion via gitattributes'
 
 . ./test-lib.sh
 
-cat <<\EOF >rot13.sh
+cat <<EOF >rot13.sh
+#!$SHELL_PATH
 tr \
   'abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ' \
   'nopqrstuvwxyzabcdefghijklmNOPQRSTUVWXYZABCDEFGHIJKLM'