When reading the "raw format" timestamp from the input stream, make sure
that the timezone offset is a reasonable value by imitating
7122f82
(date.c: improve guess between timezone offset and year., 2006-06-08).
We _might_ want to also check if the timestamp itself is reasonable, but
that is left for a separate commit.
Signed-off-by: Junio C Hamano <gitster@pobox.com>