The rot13-filter.pl script hardcodes "#!/usr/bin/perl", and
does not respect $PERL_PATH at all. That is a problem if the
system does not have perl at that path, or if it has a perl
that is too old to run a complicated script like the
rot13-filter (but PERL_PATH points to a more modern one).

We can fix this by using write_script() to create a new copy
of the script with the correct #!-line. In theory we could
move the whole script inside t0021-conversion.sh rather than
having it as an auxiliary file, but it's long enough that
it just makes things harder to read.

As a bonus, we can stop using the full path to the script in
the filter-process config we add (because the trash
directory is in our PATH). Not only is this shorter, but it
sidesteps any shell-quoting issues. The original was broken
when $TEST_DIRECTORY contained a space, because it was
interpolated in the outer script.

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