approxidate_relative and approxidate_careful both use parse_date to
dump the timestamp to a character buffer and parse it back into a long
unsigned using strtoul(). Avoid doing this by creating a new
parse_date_toffset method.
Noticed-by: Jonathan Nieder <jrnieder@gmail.com> Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>