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: NJunio C Hamano <junkio@cox.net>

git-format-patch.sh

@@ -210,6 +210,8 @@ Date: '"$ad"
 	}
 
 	mailScript="$mailScript"'
+	a\
+
 	: body
 	p
 	n