git-p4: commit time should be most recent p4 change time

git-p4: commit time should be most recent p4 change time

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: Pete Wyckoff <pw@padd.com>
Acked-by: Luke Diamand <luke@diamand.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>