filter-branch.sh: remove temporary directory on failure

filter-branch.sh: remove temporary directory on failure

One of the first things filter-branch does is to create a temporary
directory. This directory is eventually removed by the script during
normal operation, but is not removed if the script encounters an error.

Set a trap to remove it when the script terminates for any reason.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Acked-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>