When importing a repo, the time on the initial commit had been
just "now".  But this causes problems when trying to share among
git-p4 repos that were created identically, although at different
times.  Instead, use the time in the top-most p4 change as the
time for the git import commit.
Signed-off-by: NPete Wyckoff <pw@padd.com>
Acked-by: NLuke Diamand <luke@diamand.org>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>