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>

contrib/examples/git-svnimport.perl

@@ -238,7 +238,7 @@ sub pdate($) {
 	my($d) = @_;
 	$d =~ m#(\d\d\d\d)-(\d\d)-(\d\d)T(\d\d):(\d\d):(\d\d)#
 		or die "Unparseable date: $d\n";
-	my $y=$1; $y-=1900 if $y>1900;
+	my $y=$1; $y+=1900 if $y<1000;
 	return timegm($6||0,$5,$4,$3,$2-1,$y);
 }
 

git-cvsimport.perl

@@ -601,7 +601,9 @@ sub pdate($) {
 	my ($d) = @_;
 	m#(\d{2,4})/(\d\d)/(\d\d)\s(\d\d):(\d\d)(?::(\d\d))?#
 		or die "Unparseable date: $d\n";
-	my $y=$1; $y-=1900 if $y>1900;
+	my $y=$1;
+	$y+=100 if $y<70;
+	$y+=1900 if $y<1000;
 	return timegm($6||0,$5,$4,$3,$2-1,$y);
 }
 

perl/Git.pm

@@ -534,7 +534,9 @@ If TIME is not supplied, the current local time is used.
 sub get_tz_offset {
 	# some systems don't handle or mishandle %z, so be creative.
 	my $t = shift || time;
-	my $gm = timegm(localtime($t));
+	my @t = localtime($t);
+	$t[5] += 1900;
+	my $gm = timegm(@t);
 	my $sign = qw( + + - )[ $gm <=> $t ];
 	return sprintf("%s%02d%02d", $sign, (gmtime(abs($t - $gm)))[2,1]);
 }

perl/Git/SVN.pm

@@ -1405,7 +1405,7 @@ sub parse_svn_date {
 		$ENV{TZ} = 'UTC';
 
 		my $epoch_in_UTC =
-		    Time::Local::timelocal($S, $M, $H, $d, $m - 1, $Y - 1900);
+		    Time::Local::timelocal($S, $M, $H, $d, $m - 1, $Y);
 
 		# Determine our local timezone (including DST) at the
 		# time of $epoch_in_UTC.  $Git::SVN::Log::TZ stored the