t5316: refactor `max_chain()` to not depend on Perl

t5316: refactor `max_chain()` to not depend on Perl

The `max_chain()` helper function is used to extract the maximum delta
chain of a packfile as printed by git-index-pack(1). The script uses
Perl to extract that data, but it can be trivially refactored to use
awk(1) instead.

Refactor the helper accordingly so that we can drop a couple of
PERL_TEST_HELPERS prerequisites.

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