rev-list: fix --pretty=oneline with empty message

rev-list: fix --pretty=oneline with empty message

55246aa (Dont use "<unknown>" for placeholders and suppress printing
of empty user formats) introduced a check to prevent empty
user-formats from being printed. This test didn't take empty commit
messages into account, and prevented the line-termination from being
output. This lead to multiple commits on a single line.

Correct it by guarding the check with a check for user-format. A
similar correction for the --graph code-path has been included.

Signed-off-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>