perl: call timegm and timelocal with 4-digit year

Amazingly, timegm(gmtime(0)) is only 0 before 2020 because perl's
timegm deviates from GNU timegm(3) in how it handles years.

man Time::Local says

 Whenever possible, use an absolute four digit year instead.

with a detailed explanation about ambiguity of 2-digit years above that.

Even though this ambiguity is error-prone with >50% of users getting it
wrong, it has been like this for 20+ years, so we just use 4-digit years
everywhere to be on the safe side.

We add some extra logic to cvsimport because it allows 2-digit year
input and interpreting an 18 as 1918 can be avoided easily and safely.

Signed-off-by: Bernhard M. Wiedemann <bwiedemann@suse.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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