t/lib-httpd: refactor "one-time-perl" CGI script to not depend on Perl

t/lib-httpd: refactor "one-time-perl" CGI script to not depend on Perl

Our Apache HTTPD setup exposes an "one_time_perl" endpoint to access
repositories. If used, we execute the "apply-one-time-perl.sh" CGI
script that checks whether we have a "one-time-perl" script. If so, that
script gets executed so that it can munge what would be served. Once
done, the script gets removed so that it doesn't execute a second time.

As the name says, this functionality expects the user to pass a Perl
script. This isn't really necessary though: we can just as easily
implement the same thing with arbitrary scripts.

Refactor the code so that we instead expect an arbitrary script to
exist and rename the functionality to "one-time-script". Adapt callers
to use shell utilities instead of Perl so that we can drop the
PERL_TEST_HELPERS prerequisite.

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>