git-svn: fix parsing of timestamp obtained from svn

git-svn: fix parsing of timestamp obtained from svn

Ward Wouts reports that git-svn barfed like this:

    Unable to parse date: 2004-03-09T09:44:33.Z at /usr/bin/git-svn line 3995

The parse_svn_date sub expects there always are one or more digits after
the decimal point to record fractional seconds, but this example does not
and results in a failure like this.

The fix is based on the original fix by the reporter, further cleaned up.

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