format-patch: do not abuse 3-dash marker line.

Before GIT version at the end of output we used a 3-dash marker;
but 3-dash marker is special and should not be overused.
Instead, use "-- " which is a standard practice in e-mails to
signal the beginning of trailing garbage.
Signed-off-by: NJunio C Hamano <junkio@cox.net>

git-format-patch.sh

@@ -239,7 +239,7 @@ Date: '"$ad"
 	git-diff-tree -p $diff_opts "$commit" | git-apply --stat --summary
 	echo
 	git-diff-tree -p $diff_opts "$commit"
-	echo "---"
+	echo "-- "
 	echo "@@GIT_VERSION@@"
 
 	case "$mbox" in