We create a rot13.sh script in the trash directory, but need
to call it by its full path when we have moved our cwd to
another directory. Let's just put $TEST_ROOT in our $PATH so
that the script is always found.
This is a minor convenience for rot13.sh, but will be a
major one when we switch rot13-filter.pl to a script in the
same directory, as it means we will not have to deal with
shell quoting inside the filter-process config.
Signed-off-by: Jeff King <peff@peff.net> Signed-off-by: Junio C Hamano <gitster@pobox.com>