t: refactor environment sanitization to not use Perl

t: refactor environment sanitization to not use Perl

Before executing tests we first sanitize the environment. Part of the
sanitization is to unset a couple of environment variables that we know
will change the behaviour of Git. This is done with a small Perl script,
which has the consequence that having a Perl interpreter available is a
strict requirement for running our unit tests.

The logic itself isn't particularly involved: we simply unset every
environment variable whose key starts with 'GIT_', but then explicitly
allow a subset of these.

Refactor the logic to instead use sed(1) so that it becomes possible to
execute our tests without Perl.

Based-on-patch-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>