perl: bump the required Perl version to 5.8 from 5.6.[21]

Formalize our dependency on perl 5.8, bumped from 5.6.[12]. We already
used the three-arg form of open() which was introduced in 5.6.1, but
t/t9700/test.pl explicitly depended on 5.6.2.

However git-add--interactive.pl has been failing on the 5.6 line since
it was introduced in v1.5.0-rc0~12^2~2 back in 2006 due to this open
syntax:

    sub run_cmd_pipe {
           my $fh = undef;
           open($fh, '-|', @_) or die;
           return <$fh>;
    }

Which when executed dies on "Can't use an undefined value as
filehandle reference". Several of our tests also fail on 5.6 (even
more when compiled with NO_PERL_MAKEMAKER=1):

    t2016-checkout-patch.sh
    t3904-stash-patch.sh
    t3701-add-interactive.sh
    t7105-reset-patch.sh
    t7501-commit.sh
    t9700-perl-git.sh

Our code is bitrotting on 5.6 with no-one interested in fixing it, and
pinning us to such an ancient release of Perl is keeping us from using
useful features introduced in the 5.8 release.

The 5.6 series is now over 10 years old, and the 5.6.2 maintenance
release almost 7. 5.8 on the other hand is more than 8 years old.

All the modern Unix-like operating systems have now upgraded to it or
a later version, and 5.8 packages are available for old IRIX, AIX
Solaris and Tru64 systems.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Acked-by: Tor Arntsen <tor@spacetec.no>
Acked-by: Randal L. Schwartz <merlyn@stonehenge.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>