apply: remove epoch date from regex

apply: remove epoch date from regex

We check the date of epoch timestamp candidates already with
starts_with().  Move beyond that part using skip_prefix() instead of
checking it again using a regular expression.  Also group the minutes
part, so that we can access them using a substring match instead of
using a magic number.

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