A changelist that contains only excluded files due to a client spec was
imported as an empty commit. Fix that issue by ignoring these commits.
Add option "git-p4.keepEmptyCommits" to make the previous behavior
available.
Signed-off-by: Lars Schneider <larsxschneider@gmail.com> Helped-by: Pete Harlan Acked-by: Luke Diamand <luke@diamand.org> Signed-off-by: Junio C Hamano <gitster@pobox.com>