t0021: use $SHELL_PATH for the filter script

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: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>