Allow the Unix epoch to be a valid commit date

Allow the Unix epoch to be a valid commit date

It is common practice to use the Unix epoch as a fallback date
when a suitable date is not available.  This is true of git svn
and possibly other importing tools that import non-git history
into git.

Instead of clobbering established strtoul() error reporting
semantics with our own, preserve the strtoul() error value
of ULONG_MAX for fsck.c to handle.

Signed-off-by: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>