Accept dates before 2000/01/01 when specified as seconds since the epoch

Accept dates before 2000/01/01 when specified as seconds since the epoch

Tests with git-filter-branch on a repository that was converted from
CVS and that has commits reaching back to 1999 revealed that it is
necessary to parse dates before 2000/01/01 when they are specified
as seconds since 1970/01/01. There is now still a limit, 100000000,
which is 1973/03/03 09:46:40 UTC, in order to allow that dates are
represented as 8 digits.

Signed-off-by: Johannes Sixt <johannes.sixt@telecom.at>
Signed-off-by: Junio C Hamano <gitster@pobox.com>