git-rebase -i behaves better on commits with incomplete messages

git-rebase -i behaves better on commits with incomplete messages

The commit message template when squashing multiple commits is
prepared by concatenating the messages of existing commits
together.  If the messages from some of them end with incomplete
lines, this would result in a suboptimal message template.  Make
sure that we add a terminating LF after each commit message.

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