apply: check date of potential epoch timestamps first

apply: check date of potential epoch timestamps first

has_epoch_timestamp() looks for time stamps that amount to either
1969-12-31 24:00 or 1970-01-01 00:00 after applying the time zone
offset.  Move the check for these two dates up, set the expected hour
based on which one is found, or exit early if none of them are present,
thus avoiding to engage the regex machinery for newer dates.

This also gets rid of two magic string length constants.

Signed-off-by: Rene Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>