Allow 'svn fetch' on '(no date)' revisions in Subversion.

Allow 'svn fetch' on '(no date)' revisions in Subversion.

Added --ignore-nodate to allow 'git svn fetch' to import revisions
from Subversion which have '(no date)' listed as the date of the
revision.  By default 'git svn fetch' will crash with an error
when encountering such a revision.  The user may restart the fetch
operation by adding --ignore-nodate if they want to continue tracking
that repository.

I'm not entirely sure why a centralized version control system such
as Subversion permits revisions to be created with absolutely no
date/time associated with it but it apparently is possible as one
of the Subversion repositories that I'm tracking with 'git svn'
created such a revision on '(no date)' and by '(no user)'.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>