cherry-pick: regression fix for empty commits

cherry-pick: regression fix for empty commits

The earlier "--keep-redundant-commit" series broke "cherry-pick"
that is given a commit whose change is already in the current
history. Such a cherry-pick would result in an empty change, and
should stop with an error, telling the user that conflict resolution
may have made the result empty (which is exactly what is happening),
but we silently dropped the change on the floor without any message
nor non-zero exit code.

Signed-off-by: Junio C Hamano <gitster@pobox.com>