fast-import.c: stricter strtoul check, silence compiler warning

Store the return value of strtoul() in order to avoid compiler
warnings on Ubuntu 8.10.

Also check errno after each call, which is the only way to notice
an overflow without making ULONG_MAX an illegal date.

Signed-off-by: Rene Scharfe <rene.scharfe@lsrfire.ath.cx>
Signed-off-by: Junio C Hamano <gitster@pobox.com>