format-patch: make sure header and body are separated.

format-patch: make sure header and body are separated.

Since log message in a commit object is defined to be binary
blob, it could be something without an empty line between the
title line and the body text.  Be careful to format such into
a form suitable for e-mail submission.  There must be an empty
line between the headers and the body.

Signed-off-by: Junio C Hamano <junkio@cox.net>