filter-branch: use $SHELL_PATH instead of 'sh'

On some systems, 'sh' isn't very friendly. In particular,
t7003 fails on Solaris because it doesn't understand $().
Instead, use the specified SHELL_PATH to run shell code.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>