format-patch previously didn't generate a newline after a subject. This
caused the diffstat to not be displayed in messages with only one line
for the commit message.
This patch fixes this by adding a newline after the headers if a body
hasn't been added. Signed-off-by: Robert Shearman <rob@codeweavers.com> Signed-off-by: Junio C Hamano <junkio@cox.net>