filter-branch: do not forget the '@' prefix to force git-timestamp

filter-branch: do not forget the '@' prefix to force git-timestamp

For some reason, this script reinvents, instead of refactoring the
existing one in git-sh-setup, the logic to grab ident information
from an existing commit; it was missed when the corresponding logic
in git-sh-setup was updated with 2c733fb (parse_date(): '@' prefix
forces git-timestamp, 2012-02-02).

Teach the script that it is OK to have a way ancient timestamp in
the commits that are being filtered.

Signed-off-by: Junio C Hamano <gitster@pobox.com>